We have around 20 some odd years of lore for MTG. I was wondering if you guys can help me compile information that is common to all planes. I know for a fact that each plane has leylines, and that magical fonts tend to originate near such leylines, along with many other strange effects. Each plane also has elementals/horrors/Pure magical beings with varying power levels. Finally, generally speaking anyone can learn to use magic if taught from early in life. But I do not know if anything else is common.

Leylines
Magical Beings
Anyone can use magic
Plane Souls
Distance and Position are relevant in the Blind Eternities
Raw Mana is generally bad especially in large amounts

Kaladesh doesn't have leylines

Did Nissa confirm that in a story? I thought it sort of did, in a sense that there were major paths that aether flowed in before spiraling away into various eddies.

If a plane has a soul it has leylines, Nissa confirmed that I think.

Its more like aetherflows. Remember, aether exists mostly in the air in Kaladesh

That doesn't mean it doesn't have leylines. The only reason it wouldn't have leylines is because the plane is dead or eldrazi consumed.

Not really. This is a bit of a false inference.
For a long LONG time they didn't define what leylines were, and they don't actually go into what they mean even once they did.
The only worlds they've explicitly given leylines to are Ravnica, Innistrad, Zendikar, and Tarkir. Dominaria probably had a mention of it somewhere, but they weren't significant.

I'd also question the omnipresence of planar souls.

Well, there are planes that are just empty planar voids without mana, matter or life, so none of the things mentioned in the OP is technically true for all planes...

Yes. Remember, the only planes we frequent are those that can be interesting for a card game.

Iquatana, for example, is pretty hostile to life as we know it.

I think it less false inference and more we don't know enough to say anything concrete on leylines. Also denying while questioning that very plane has a soul is fine, denying that planes that do have matter and life on them don't have souls is not. Still, we cant say anything until Nissa, the one who talks with worlds, visits a sizable number of worlds

The fact of the matter is, we have 2 planes that have confirmed possible worldsouls, and Lorwyn, which might have a LOT because that spider thing clashed with all known everything on the matter. (or can we all stop pretending that keeping Ms. Nissa goes to Lorwyn was a good thing? They already retconned the pants off her, spillover onto the worlds themselves is too messy)

So we have even less inclination to believe that than we do the leylines.
There are slightly more nebulous cases like Progenitus and Animar, but... they don't seem to fit what we know of the likes of Gaea. A little more like Ashaya (as long as you don't lose track of her )

The original Ley Line card was from Mercadian Masques. #WOTCstaff

Something I thought of this morning, you can basically guarantee every plane we visit will have a walker native to that plane. You could very loosely infer that every plane has a native planeswalker

Yeah, the formula pattern I see roughly moving forward is: 2 Gatewatch members, 1 Native 'Walker, 1 Returning non-Gatewatch member. Plus or minus here or there to taste.

Lorwyn has no native planeswalkers.

Pre-Sundering Alara did, yes?
So far no examples of Post-Conflux Alara planeswalkers.

I would believe plane souls (maybe not always embodied) mana, and some FORM of leylines, even if they don't behave as is common. The likelihood of a native planeswalker is also seemingly multiversal.

If we revisited lorwyn or any older planes they would make a native walker for it.

Lorwyn might be problematic. We have VERY unconventional elves, mayfly faeries good for a year or two, unpopular kithkin, created mana beings, non-legged merfolk, giants, and mildly unconventional goblins.

So... pretty much boggart or bust out of those options.

Flamekin are "born" they they can have a spark.

Flamekin are created by the elemental of Mt. Tanufel.
They're also mana beings.

That said, I'm aware that Doug doesn't seem to know this and answered that they could have Sparks.

I believe that he was aware, since he did conceive them.