There are 2 things I am afraid of in ARAM, Diamond+ and Unranked...
I'm afraid I fall into the latter category... Far more ARAM games than normals (and yet I somehow have something like 15% more wins than losses)... And getting placed against diamond players in ARAM isn't uncommon... But yeah, something to distinguish those that spend altogether too much time in that game mode would be nice. I imagine the same could be said for Dominion. In my case, my performance there is the fact that I play mid almost exclusively so my champion pool is more or less entirely mages.
Some sort of recognition of how much of a danger a player is likely to be on alternate game modes would be good though... Maybe ARAM gives little dots at the bottom and Dominion gives marks on the side of champion portraits? Just spitballing here...