I remember talking about the idea of having instants and sorceries having subtypes, such as a subtype for feats of marksmanship or pyromancy.

Heck, having marksmanship "tribal" with Open Fire and Arrows of Justice seems to be quite fun. (I'm a degenerate spike, but I think this would be cool for flavor-driven environments like Commander and especially casual.) I wonder if we can make a database containing what we could do with this.

Ah well, I think it would be too ambitious for too little payoff.

Perhaps I could make a thread in Constructed about this.

Such a system would have to be built into the game from the start to make any sense. In the absence of tribal effects, it's just adding meaningless words onto everything. And if there are enough tribal effects that the subtype really matters, they'd need to take that into consideration when balancing the cards. And the subtype wouldn't be adding much that wasn't already present via the card's color.

arcane was a instant/sorcery subtype with tribal support

If you could adopt the practice going forward giving all instants/sorceries subtypes in the same way you do creatures, and then sprinkling support for arbitrary subtypes here and there, then that would give the game some additional depth and a lot more synergy density which is generally a good thing. The downside would be complexity, wordiness, and other things that make the game harder to learn or harder to understand at a glance.

Personally, I think its a good idea. The bottom line for magic is that its a game with a steep entry curve, and much of RnD's focus has been on alleviating that I think, and that's probably the biggest reason to shut this down, but I don't think that an accessible game is necessarily a good game (although its probably a profitable one), So while I do think instant/sorcery subtypes would make the game marginally more difficult to get into, which is a significant problem when your game is already difficult to get into, I think that the benefit it brings to the game is bigger than the cost it adds.

Although I might be missing some angles.

as an example

Flaming Bolt -
Instant - Fire
~ deals 2 damage to target creature or player.

Flame-eater -
Creature - Imp [C]
Whenever you cast a Fire spell ~ gets +1/+0 until end of turn.
2/1

Arcane was a block mechanic. It didn't require any changes to other sets. It also wasn't a huge success. There wasn't much connecting the different Arcane cards, so it mostly meant "if you want to play with splice or spiritcraft, you're limited to this small set of often overpriced cards".

The only other spell type, Trap, was also a block mechanic. For the most part, the subtype just tied together the various "cheaper if certain conditions are met" cards, but there were also two cards that interacted with them specifically. I could see them making more of this type of thing in the future.

What I don't think they will or should do is try to broadly apply spell types to instants and sorceries like they currently do with creature types. There are several key differences between the two.

One is that, when done right, creature types are almost invisible. You can often already guess from the name and art what the creature type should be. Even without knowing about tribal effects, it's clear that a creature card representing a goblin should have the Goblin creature type. With spell types, this is going to be much less clear, especially since none of the existing instants/sorceries were designed with that sort of categorization in mind. Just what type of spell is a Cathartic Reunion?

Also, creature types are versatile. There are many ways to Goblin, so there are many ways to build a Goblin deck. On the other hand, it seems that most spell types would be closely associated with mechanics. Pyromancy would just largely overlap with "red burn spell", Necromancy would just largely overlap with "black reanimation spell", etc. That would both make it hard to give a set enough of a concentration of a given spell type to allow tribal interactions to happen (there are only so many black reanimation spells a set can have) and hard to build a deck around it. On the other hand, if they're defined more loosely, such that, for example, any card associated with death, vampirism, the undead, or corpses counts as a Necromancy, it would then largely overlap with "black spell" (a bit of an exaggeration, but you get my point).

Another issue is that creatures are just much more interactive. There are all sorts of ways to make tribal creature effects: you can boost their P/T, give them abilities (and there are so many options there), and you can have abilities that trigger when they enter the battlefield, attack/block, deal/are dealt damage, and die. Interactions with instants and sorceries are much more limited. In particular, interactions instants/sorceries can have with other instants/sorceries are very limited. So it would mostly be an A/B mechanic, where you separately need the spells of a given type and the cards that care about that spell type, making it even harder to build around.

Just consider: in the current Standard, there are 700 cards that mention "creature" in their rules text, but only 15 cards that mention "instant" or "sorcery" (if you exclude the use of those words as timing restrictions, like in embalm's reminder text).

Spelltypes can also be invisible when done right. You can probably guess that flaming bolt would be a fire spell, if fire was a subtype. Creature's subtypes are often also not guessable, especially class types, but races can be ambiguous too. I don't think this is necessarily a bad thing, though. You want your typeline to be cohesive with the rest of the card, but creature type is not something you need to constantly know.

Catharatic Reunion can be whatever you want it to be. You could call it an "event" spell. Backwards compatability isn't really important though. When they did the creature type rework a lot of old cards were errata'd with creature types that didn't make sense, but for the most part it worked, and in the places where it didn't it wasn't a big deal. Beyond that, I wouldn't propose errataing subtypes onto all old cards anyway, since that has major balance implications. I don't think assigning subtypes to spells is innately more difficult than assigning subtypes to creatures.

Types are only as versatile as you want them to be. Beasts are WUBRG, vary heavily in CMC, effect, and body. If all you know about a card is that its a beast, you can't guess very much about it. On the other hand, if something is an angel, you can guess that it is almost definitely white, that it almost definitely has flying, and that its CMC is almost definitely 4 or higher (probably higher). Goblins are mostly low CMC, geared towards aggressive strategies, have higher tribal support than most creature types, are primarily red, etc. Subtypes for both creatures and spells can correlate with specific trends (low cmc, high cmc, high tribal support, low tribal support, tends to have specific effects, etc.) as heavily or as loosely as you want them to. I don't think that spell subtypes and creature subtypes are different in this way. I don't follow your argument on spell subtypes being innately narrow or over-encompassing. As an example, these subtypes could be common examples of black magic: "Death, Unholy, Arcane, Trickery, Ritual, Mental, Forbidden".

Your third point is valid, I feel. You can have an elf that cares about elves and that's all fine and dandy, but there's a limit to how many fire spells you can have that themselves care about fire spells. This is a limitation of subtypes on sorceries/instants, but I don't feel that its a weakness so much as it is a lack of a strength that creature types have. Ultimately, I think there's still plenty to justify widespread subtypes on instants/sorceries, but not a lot of drawback to implementing them.

I don't think they'll ever implement them though.

I did this in a pokemon set I was going to make.

The current race/class system hasn't been around forever. Ditto for the idea that everything, including artifact creatures, legendary creatures, and self-animating permanents should have creature types. And some of the early creature types were completely useless (see Avizoa and Barishi, for example). However, the idea of creature types and tribal effects involving them have both been around since the beginning. The Grand Creature Type Update certainly introduced some weirdness in the corner cases, but most of it was straightforward. I don't think the same could be said of a potential Grand Spell Type Update.

For example, here are the black instants and sorceries from Ixalan:
Boneyard Parley
Call to the Feast
Contract Killing
Costly Plunder
Dark Nourishment
Duress
Grim Captain's Call
Heartless Pillage
March of the Drowned
Skulduggery
Spreading Rot
Sword-Point Diplomacy
Vanquish the Weak
Vraska's Contempt
Walk the Plank

What spell type should each one get? And would anyone else reach the same conclusions?

I just don't think there'd be the critical mass to make spell types actually matter. In the absence of that, the extra wording is just clutter and confusion without benefit. For the most part, just restricting things to "instant or sorcery" is already narrow enough. For extra flavor, a color restriction can be added.

(There's also the issue that many potential spell types feel like they could also fit on enchantments or even creatures. For those, a supertype would make more sense. But even then, it would need to be part of the design to start instead of being retrofitted in.)

Melee
Skulduggery
Swift Kick
Martial Glory
Prey Upon

Necromancy
Boneyard Parley
Vile Rebirth
Grim Captain's Call
March of the Drowned
Lingering Souls

Signal
Call to the Feast
Trumpet Blast

Crime
Contract Killing
Heartless Pillage
Walk the Plank

Punishment
Vanquish the Weak
Public Execution
Arrest

Threat
Duress
Threaten
Sword-Point Diplomacy

Psychic
Inquisition of Kozilek
Pacifism

Counsel
Counsul of the Soratami
Diabolic Tutor

Divine (high synergy target)
Divination
Divine Favor

Research
Inspiration
Brainstorm

Electric
Lightning Bolt
Lightning Strike

Earth
Vraska's Contempt
Boulderfall

My sights are mainly aimed at Limited when coming up with the idea though. Many spells that have to do with marksmanship are only viable in limited, so it would be nice to have a constructed card that explicitly synergizes with them.

Ampryn Marksman
Creature - Dwarf Archer
When ~ enters the battlefield, search your library for a Marksmanship spell and put it into your hand.
Reveal a Marksmanship spell from your hand: target creature attacks this turn if able.
1/2

But I think all of these can be put into their own type. A tribal theme would be a lot harder with current MtG worlds.

i think itd be interesting so long as no widespread errata is involved

cause errataing 1000s of old cards is dumb