I can tell! Its very complicated along those lines as its almost trying to get my player to pick and choose what he wants.
That's working as intended.
If he wants a creature or if he wants, essentially, environmental effects. How would that work short term versus long term, in the sense of evothaum. Is there a particular element or power he should take to help him?
Well, the sponsored magic version would be more complicated but also more versatile. You can do anything with it, as long as you can describe how your magic can do it.
The modular abilities version is more work up front and less versatile, but it is a lot easier to use, since you can avoid the magic rules altogether.
With the evothaum version, you'd basically just say "I play this trap card, which binds an opponent when he attacks me", which would basically just be an evocation block without any fuss. But he could also just solve a lot of problems with magic by having the right cards available. It's not so much about any element, the deck of cards would have its own sort of magic, outside of the standard elements.
For the
sponsored magic version, you'd just need to take sponsored magic (deck of magical cards) and think up an agenda for the deck. Maybe a former magician resides in the deck, maybe it was created by someone that wants it to be used in a certain way, etc. That agenda can be used to deal with debt. You could also think about using different skills than the regular ones. Performance would for example fit a deck of cards from a magician.
For the
modular abilities version, we'd need to set up the modular abilities first. I'm not sure at the moment how much refresh you'll play with, if it was mentioned earlier, I couldn't find it. Let's stay low and assume a feet in the water campaign.
Since the deck of cards is an item, we can go with an item of power discount, albeit a small one. The basic write up would look like this:
Deck of Cards (Item of Power) [-4]
Modular Abilities [-2]
3 Form points [-3]
small/concealable [+1]
So having the deck like this costs 4 refresh. The next step would be to list the cards. I'm not too familiar with Yugioh, so I'll just go with something generic.
Small red Dragon [-3]
Breath Weapon (fire) [-2]
How to tame your dragon (Stunt): Use presence to use breath weapon, when you can order your Small red Dragon to set something on fire.
So any time you draw the "Small red Dragon" card, the 3 Form points from "Modular Abilities" get filled with the powers and stunts on the "Small red Dragon" card, listed above. What happens in the game is that the character draws and plays the card, a small, red dragon appears, and he can order it around to do stuff, which will mostly be setting things on fire. This under the assumption that you will do a lot of summoning and your presence skill would be better than your weapons skill, which seems appropriate for the kind of character you describe.
Now you just need to create a list of cards he has in his deck. All of them can have powers worth 3 or less refresh, and he can only ever use one of the cards at a time. Before he can use another card, he would have to dismiss the old one and draw the new one, which costs him an action (it's what "changing your powers" takes under modular abilities). You can also add new cards throughout the campaign, whenever you feel like it might be appropriate. You don't even have to look for a reason, it's a card he always had in his possession, he just didn't use it before. That way, you don't have to have everything written out up front, a good deck can have quite a lot of cards, I imagine.