Some inherent map flaws keep Dominion and Twisted Treeline less fun than they could be due to balance issues.
Riot is tuning balance specifically for those maps to make them fun to play, but not tamper with the balance of the classic Summoner's Rift. The way Riot is planning on doing it is by making champion-specific changes to the champions only on those maps. Theoretically, this would mean "passives" that apply to champions deemed overpowered or underpowered on the various maps that aren't Summoner's Rift
RiotNome discusses further on the matter:
“
Dominion’s fun (it’s fast). So is Twisted Treeline. But they both share one key problem – League of Legends is competitively balanced for Classic gameplay on Summoner’s Rift. Our original goal was to create rulesets that would result in balanced experiences across all game modes with the same champion stats. We’ve experimented with itemization, game pacing, map features, and other design elements. While we’ve had limited success here, there’s still room for improvement, so we’re going to take the plunge: in the near future™, we’re going to be implementing map-specific balance changes for individual champions.This is going to take a few patches to implement, but we’re actively working on the tech, and we’re already got a bunch of specific balance changes in mind that we’re iterating on. We’re sure there will be lots of questions regarding the initiative, so we’ll announce more specifics when the time comes. I’ll actually be joining the live design team to head up map-specific balance for TT and Dominion. Remember that this is just the foundation for support - we’ve got some other tentative plans that we’re exploring to enhance both maps farther down the line!"
Source - Riot Games