While unpopular to say, there's a simple reason why it's not coming back: It's not a MOBA-game mode.
Before you bite my head off: Dominion was cool but represented a drastically different thing. Champion strengths were different in that mode because of what was required. Tower-and-objective taking weren't a thing. Certain champions didn't even have a passive (or sometimes an ability depending on the severity) due to how the game mode worked. It was nothing like League of Legends. Now, it COULD have worked had Riot put down everything about SR and designed releases and balance around it...but it didn't. The game mode was just too different and players who wanted a CP style game would find it elsewhere.
Riot's also not the only one who made this mistake. SMITE had a 4v4 map with a similar goal and that was shut down for the same reason. I believe it exists in DOTA2 as a custom game option but I imagine the playerbase is half-a-handful at best.
Nexus Bltz, by comparison, is still League. It still has aspects of SR and champion design isn't so far removed that champion abilities are broken or obsolete (With the exception of Ivern's passive for some reason and Shyvana passive) but it offers the same gameplay, just at a faster rate. Now, what I'm going to propose will be drastic and might come off as abrasive but...
Make your own Dominion.
I'm serious.
Build a game, from the ground up, to support Dominion. Character design around it. Make aspects specifically for the mode. Recreate what you love about dominion but focus all your effort into making it the best dominion it can be. After all, that's where League came from and look where that got Riot. If Dominion is built by dominion players for dominion and nothing else, it has the potential to be great.