Theres Clash Royale which provides amazing PVP gaming, and is going to get even better with the two player idea. But- theres a lack of multiplayer games to the same calibre. I mean why can you not play a dungeon with a friend,or some people online? Sometimes you want a good PvE(Players versus environment) game. Instead of competing against each other, you have a nice game trying to achieve objectives as a group. An accessible game like Clash Royale but playing as two people versus dungeons might not be bad. There you go developers- idea for you. Play with a friend in 100 levels of 'instanced' dungeons with challenging difficulty. Fight a dragon every 10th level. Get loot.
Have you looked at Dungeon Hunter 5? It might not be as accessible / casual as Clash Royale, but seems to otherwise fit your description. We're working on a couple casual multiplayer games, but... that's all I can say about that right now
