To help in the organization of this process i think it would help to come up with categories for the creature types first then fill in...
So for example, Elementals (I would suggest the classical) Fire, Earth, Air, Water
And then of course you have to have the ALI sectors, Life and Death, Order and Chaos.
Beyond that I would suggest cross referencing those sections with different categories to ensure balance.
i.e you need Offensive and Defensive (creatures), Offensive and Defensive (Spells), Single target vs. Multi Target (for Buffs/Debuffs)
Beyond that I think you could add Artifacts, Hero Equipment(maybe one playable per round, stacking)
From this we can fill in so for example fire will have more offensive capabilities but less defense etc etc etc. Oh also I would avoid making "super" items or creatures, make everything fair and level (as much as it can be when you have to buy cards) For an RPG game it should be about skill of the player not who can buy the most cards and make their deck uber 733t R0XX0RZ
