yes and no. Most of hearthsotnes cards come from World of warcraft, and there is a good set that are based on the game and were never actual characters on the original game. Similarly Riot has a theme and ambiance already on place. You have one single Troll in game but you know there are tribes of it, several in fact, you could make a bunch of different cards based on that. You also have some of the lore like the ruined king and the bligewater that wouldn't make much sense on champions but woudl work wonders on cards. You could use current and discontinued items as well and include passives and actives of champions and cards as standalone cards/support cards. Finally you have all the material left from the retcon, with minor touch ups a bunch of secondary and background characters could make it out, you only see the most prominent generals from demacia and noxus, doesn't mean those are the only ones.
If they are worried about balancing work they could do what small card game firms do, do a big update once (or twice) a season and leave the game pretty much unchanged for the duration, if a card happened to be too overpowered you either ban it for competitive play or hotfix it.
In fact if Riot sells me a license cheap I could make one game myself and I can gurantee you it will sell well on the apps store for mobile play.
wat
