First - congrats on getting this up and running! It's not perfect, but it's a start.
I can't remember if this was destroyed but the great Forum Fire of Twenty-Thirteen, so I figured I'd post it for effectively the first time.
Something that Magic has never done, but always feels like it wants to do, is have gold and money. There are many cards (like Greed) that depict money, or cards (like Gwafa Hazid, Profiteer) that depict the use of money, but nothing that "is" money. Usually the closest the get are allusions to life or card-draw... which is weird, because you not all spells spend life or spend cards through discarding, but almost you always are spending mana.
I think there’s room for such an effect. Players should be able to either spend gold instead of mana, or spend gold in addition to mana. Previous attempts have run into problems, because the gold has always been treated as mana of any color. What if gold shared colour identity just like mana? Then you could have white gold, black gold, green gold, phyrexian gold, snow gold, etc. The main benefit of this kind of system is that it prevents gold from twisting each color’s identity.
All colours would have access to gold, but I think some will be more inclined than others:
White – Uses some gold, but either distributes it or the effects evenly. Very likely to tax all players’ gold.
Blue – Very likely to use gold. Blue would care about the movement and tracking of gold. Most likely to change gold’s colour, or change things into gold.
Black – Very likely to use gold. Black would want to hoard gold, spending it only when absolutely necessary. Will also steal gold if given the chance.
Red – Not very likely to use gold. Red sees gold as something to burn rather than spend, or punish others for collecting too much of something that should only ever be temporary. If Red does get gold, it should be used right away.
Green – Not very likely to use gold, as it is an artificial construct of civilization. However, if it does use gold, it will make it available to everyone.
Here’s a few card-designs that use gold:
Goblin Larsonist
Creature – Goblin Rogue {C}
Whenever Goblin Larsonist deals combat damage to a player, that player loses a gold counter. If they do, deal 1 damage to target creature or player.
1/1
Trading Post
Artifact {C}
,
: You gain 1 gold of any colour.Paradise Infusion
Sorcery {U}
Draw cards equal to the amount of gold spent to cast Paradise Infusion.
Ruthless Debtor
Creature – Zombie Advisor {C}
At the beginning of your upkeep, take a gold counter from each opponent.
2/3
Elvish Prospector
Creature – Elf Rogue {R}
: Each player gains 1 gold counter of any colour.1/1