You mean other than:
1. It would significantly increase the cost of developing the game
2. It would draw development resources away from other parts of the game
3. It would change the focus from a clash of empires to a clash of fleets
1. Yes, it would. I don't see the problem in that. It would at the same time, expand the sales potential of the game. Remember, each GC game has had pretty good sales potential and the budgets have been progressively increasing as the title has gotten more popular.
2. I fail to see how this is a problem. I do not see any major shortcomings in the strategic aspect of GC2 other than what I described earlier.
3. To some extent, all games are already like that. They are about a clash of ships. Whether you see it in battle viewer or whether you control it, it's already a clash of ships.
Of course it would detract from the strategic aspect. There's only so many developers, and any time they spend making tactical combat happen is time not being spent on the other stuff. Especially if you want to cram an RTS in, since that's an entirely different beast from a TBS. (Not to mention the major pushback from existing fans about suddenly finding an RTS in their previously turn based game.)
There's a reason why the economic and empire managmeent side of a game like Shogun 2 is simpler than in Civilization.
As indicated, it would increase the cost of the game, but at the same time, it would increase the sales potential. As for pushback, it would come down to how well implemented the system was. If it was poorly implemented, then yes, I would expect a backlash. But if it were well implemented, well, I think it would have the potential to greatly add to the game.
I'm calling for a level of sophistication comparable to GC2 in the strategic layer with tactical combat, not a "dumbing down" - heck they could re-use some of the assets from GC2 where appropriate.