My game has cards that can be played later. I am considering having the option of merging two character's together but I am concern that it would create additional costs to making the game (a combined character pawn) plus the idea is very Yugio-esque so I am concern that I may be in legal trouble for that idea. I have not played card games besides Yugio so I am not sure how original their games are.
Card elements
You're referring to fusion summons, I presume, rather than sacrificing two monsters to summon a LV7+ monster there?
Regardless. I wouldn't worry too much about the legal issue of this, unless they have a patent on it (which would be unlikely seeing as it's essentially the same as any other paying one of each of two things to get a third thing [The obvious example of which being buying a road in Settlers for a brick and a wood]), it's just a mechanic weather it's been used outside of the context of Yu-Gi-Oh or not. Provided the game isn't a Yu-Gi-Oh clone, of course, in which case there may be issues. I can't say I've actually seen anything exactly like it outside of Yu-Gi-Oh in board or card games, however, although it's not unheard of in videogames. (Permanently in the case of Jade Cocoon, temporarily [I think] in the case of Soul Nomad [Sorta for the latter. Very, very sorta]).
Mechanically speaking, though, I don't think it's actually distinguishable from any other 'pay X and Y for Z' mechanic. (Heck, if I'm wrong and you did mean the sacrificing of two monsters to summon a LV7+ monster, then it's pretty much indistinguishable from paying X to get Y)
Standard I Am Not A Lawyer disclaimer applies.
As usual with this sort of question (which we gat around here a lot), I'm going to advise you to not worry about it. Startup game developers aren't worth the legal expense of suing, and it would take an awful lot to convince anyone that you're lifting intellectual property from Yo-Gi-Oh simply by using a mechanic. Mechanics in games are basically unprotectable, since they are an abstraction, and you're manipulating abstract concepts. It is, indeed, getting C by paying A and B. How many other situations can you name that are similar to that, in games and real life?
Don't let the modern culture of hyper-legalism get to you. Unless specifically protected (by a copyright or patent), everything's free game, and even "protected" material can be used within reason (Google "derivative works"). I'm not saying make a clone of Yo-Gi-Oh, just not to worry about trifling details. Magic the Gathering had dozens of complete and utter clones running around, and Wizards never bothered to sue any of them, but rather let them sink under their own dead weight.
People don't want to play the poor relation of a popular game, and that's all the protection most popular games need.
Thanks for the response - it definitely is encouragement to do what I imagine as the opposition!