Before you mention Gears Tactics, that's more like XCOM. It isn't real-time strategy, and instead is turn-based.
I honestly think a Gears of War spinoff that plays like CoH would fit like a glove. Just imagine micromanaging multiple squads of Gears or Locust at once, trying to make use of whatever cover is available while the battlefield keeps getting blown apart, and picking doctrines to define your playstyle in a match. I'd also favor the real-time tactics gameplay of CoH over something like Age of Empires or Starcraft, as economy management and base macro doesn't really fit the Gears universe.
Doctrines would also be really good to represent the various COG and Locust armies. For instance, if you're playing as the Locust, you could have a "doctrine" that allows you to create Emergence Holes to pop up on your enemy's flanks, while calling in Seeders and/or Corpsers. Another "doctrine" could favor making use of Boomers, Berserkers or Brumaks to overpower your enemy's defenses. Meanwhile, the COG could have a doctrine that favors fortifying key locations with rapidly-deployable defenses, or another that favors the use of King Ravens to reinforce and relocate your troops while calling down airstrikes.
I'm sure there's more that could be done, but those are just rough ideas. Really, I think if the developers behind Gears of War were to try this, we could get something good. It may be niche, but it's a niche that I think would be great for GoW.