How would people feel about getting +1 to all ability levels upon hitting level 25? Right now after 14 there's little to no ability progression and you just get XP as a side-effect of farming gold but there is no goal to getting levels anymore aside from stat growth. Or maybe a reward like the kongor rewards (choose between 50 movespeed, 15% damage output, or +20 to all stats). Maybe automatic SotM boost? (only if everyone has a staff of course). Lot's of possibilities.
It's just an idea, and it doesn't have to be any of these, but I do feel like it could make the game more interesting if there was a significant reward upon reaching max level. In doto when you hit level cap you unlock all talents and it's massive, and I know talents aren't happening in HoN, but it does give you this goal to work toward.