If Heimer is such a problem in Dominion, nerf him. In Dominion only. Surely there can be a numbers modifier for specific champions? You have hidden passive debuffs when specific conditions are met, why not have a condition be "this is Dominion" like the Dominion/Proving Grounds buff is, and just flat out change Heimerdinger's numbers?
Would you rather Riot spend a year (quite possibly more) building all-new versions of each character that are maybe balanced on Dominion but most likely not OR make map/aura/item changes that potentially adjust champion balance into a bit better place?
Stuff I've seen considered by reds, such as a cheaper version of a deathcap or a more caster-friendly aura (different way of doing mana regen as well as maybe some free AP) would be small and doable by a small team, and potentially make some big changes to Dominion viability - in my example hopefully it'd make mages be less useless xP