I think it would be cool if you could earn Tokens or Troop cards (for newbies) or Writs or something else for playing Arena.
The lack of Hero XP really does need to be made up for, somehow, and Trophies just don’t really cut the mustard (being earned from pretty much every other part of the game, and having very marginal, long-term benefits).
The other rewards from Arena, not counting Arena of Valor, can all be earned in other places at least as efficiently, if not more so. It just seems like it’s lacking a main draw card, aside from that occasional Orb of Chaos etc. from the Arena Events.