The thing is, diplomacy, espionage and the such are absolutely essential to the strategic aspects of the game, otherwise known as the reason people play this game.
Espionage as implemented in GCII was about as essential to the strategic elements of the game as mercury is to your health. It was little more than a money sink that provided little worthwhile information about your opponents and carried an excessive price-tag, and which was little more than a nuisance in most instances (except for the spies spawn everywhere event, which was a pain rather than a nuisance).
As for the actual subject of the thread: in my opinion, an implementation of tactical combat which is fun is superior to autoreslve and the little movie that played in single-player mode. As long as there is a functional autoresolve that normally does not produce significantly worse results than the player would attain by playing the battle themselves (assuming that the player is of "average" skill), then I don't have a problem with using autoresolve when I'm not inclined to control the battle myself (e.g. 20 battles in a turn - I might pick one or two, but I probably wouldn't control all of them, or a 'battle' of 200 state of the art battleships against a handful of obsolete fighters).
Concerns about multiplayer don't bother me since I'm not interested in it, but I fail to see why you couldn't have a game setup option that requires all combats to be autoresolved, or which limits the players to one or two manually controlled battles per turn with a configurable time limit (configured during game setup) for each battle, which as far as I can tell would make the issue with people taking forever resolving each and every little fight that comes up into a nonissue. These are options that games have had for years already, and while it may not be the absolute easiest thing in the world to program in, it certainly isn't the hardest.
Regardless, neither the presence nor the lack of tactical combat would materially affect whether or not I give the game a chance, as whether or not the game is fun isn't dependent on the presence or lack thereof. I enjoyed Age of Empires, an RTS with real-time tactical combat, Sword of the Stars, a TBS with real-time tactical combat, Warlock: Master of the Arcane, a TBS with an implementation of autoresolve, and Civilizations II, a TBS with a different implementation of autoresolve - as long as the game itself is fun and enjoyable, its combat mechanics don't matter to me. Nor would an announcement regarding either the presence or the lack of tactical combat signal the end of the world and the ruination of the game to me, unlike it appears it would to some of the people commenting in this thread.
When I get a game which is a sequel to a game that I've played before, I hope for an improvement in the feel of that game. The way GCII handled combat is one thing I felt could be improved about the game, and I don't think that changes there would significantly impact the feel of the rest of the game unless the developers decided that they were going to change the focus of the game from the strategic side to the tactical side, which I feel is rather doubtful. On the other hand, it's also not the only thing that could be improved, and it's certainly not essential to me that it receives changes. What would disappoint me is if GCIII turns out to be GCII but with 'better' graphics (and which would have 'better' graphics would be a matter of opinion - what new games have isn't always an improvement, at least not to everyone; maybe I'm unusual, but I like the graphics of Morrowind better than those of Oblivion, and no, I'm not referring to modded versions of either Morrowind or Oblivion, nor am I only referring to the appearance of the characters), because that's not what I'm looking for in a game. Pretty visuals are nice, but they're worthless if the game isn't fun, and they aren't worth the cost of a new game if they are the only difference between the new game and a game I already have.