What Dominion needs is a ranked queue, something Riot originally "promised" to look into when the mode launched, but nothing ever came of it. Support for Dominion tanked for the same reason Riot provides no support for Twisted Tree or Howling Abyss: eSports. The name of the eSports game is Summoner's Rift, and any resources allocated to Twisted Treeline, Crystal Scar, or Howling Abyss are resources that aren't being allocated to Summoner's Rift.
Before the Dominion playerbase crashed, there was a bot -- called Jabebot -- that could access and display the normal mode match-making ELO for Dominion. Send a tell to Jabebot, and it would reply back with the ELO of the members on your team. This created a de facto ladder that provided an incentive for "serious" players to keep playing. Draft mode allowed players to ban the most obnoxious/broken/ZOMGOP champions (overwhelmingly Kassadin) and the playerbase was mostly satisfied while it waited for Riot to create an official ranked mode.
But that didn't happen. Instead, Riot destroyed Jabebot and the playerbase slowly crumbled. I don't have the official numbers, but Riot does, and I'd bet my ass the drop in players queuing up for Dominion (specifically draft mode) perfectly correlates with Jabebot being dismantled. And this is one case where correlation DOES equal causation.
So here we are ... some 2 years later, and still nothing.
The solution is the same as it ever was: give Dominion an official ranked mode. Give players -- the Dominion fans in particular -- an alternative to Summoner's Rift (and to a lesser extend, Twisted Treeline) for unlocking ranked/seasonal achievements, and the playerbase will return (and I'm one of them). Not everybody wants to grind Summoner's Rift just for the victorious ward/skin or border. Ranked Dominion can easily support the same divisions Bronze V through Diamond I. Challenger isn't necessary for Dominion, because all the professional tournaments will be Summoner's Rift. Hell, Riot could just "cap" the Dominion bracket at Platinum I and assign the same rewards as Summoner's Rift.
Alas, this is unlikely to ever happen. Because lolesportslol. Riot -- or perhaps, more sinisterly, Tencent -- has put all its eggs in the Summoner's Rift esports basket. So the other modes will suffer because if you're playing Twisted Treeline, Crystal Scar, or Howling Abyss, you're not playing Summoner's Rift. And Summoner's Rift is where all the money is.
"Follow the money" -- it's as true in video games as any other business.