I like the idea of just converting Mana into [Modular +1/+1 counters] or [0/0 Artifact Creature — Myr with Modular 1] as a sort of basic ability/spell. And then any other abilities are paid for with a combination of Mana and Modular counters.
So, for instance, there'd be a 'Copper
' card. Once I had access to 2 mana, I could begin spawning Modular 1 Myr with the [tap to add 1 mana] ability, or convert a Modular counter on Myr with 2+ counters on it to that ability. (Although I'd probably end up using Alloy
instead of copper, 'cause I have no real idea how the colors work and it's easier to just stick with everything being colorless.)
I have no idea how you would need to balance this, especially with 1 cost cards like this
. Also, a Modular 1 Myr Servitor is probably the most broken thing ever, no matter how much I want it. Maybe as an endgame/campaign ability?
Character advancement would be though gaining new abilities to add. Obvious plot hooks would be tracking down things to pull them apart for their abilities. What little I know about the lore is that some guy called Memnarch created the Myr, so MC is probably someone who saw the potential of the Myr, but decided she could do better, so she started ripping them and Arcbound stuff apart to figure out how to make her own. If we go down that path though, they're probably not 'true' Myr or Arcbound things.
If MC is the designated 'colorless' character, could also serve to highlight that distinction.
Anyway. While I think this idea is cool, I have no idea how well it would work with your plans (my guess is poorly). Hopefully it gives you some ideas though.