My issues with Mecha are that even in fantasy Sci Fi, they're only for ground combat. Unless we want to go full-on StarWars where physics means absolutely nothing and we can do completely ridiculous things that are only possible in someone's imagination. Giant Robot combat in space falls into this category of pure fantasy.
As to mecha for ground combat, until we allow custom unit builds for ground combat (like we do for spaceships), anything more than them being fluff in a tech that gives a generic bonus isn't going to happen.
I'd be perfectly happy if we had a Tactical Ground Combat system (in fact, I'd rather have one of those than the long-discussed Space Tactical Combat system), because it adds a significant dimension to the game that is currently very much lacking. But even there, you'd have to be abstracting up - "units" wouldn't be individual things, they'd be conglomerations: battalions, regiments, and divisions of things. And I suspect that there would be very little point to building the level of customized Mecha that we do for ships in such a case. More, you'd be selecting for certain generic characteristics (speed, firepower, armor, maneuverability) of the group as a whole, rather than pimping out a specific mecha design.
Anything more and we're back into a full-scale RTS-style game in the middle of a 4X, and I think everyone knows how I feel about that situation.