Most of my concern comes from Firaxis' previous sequels and remakes over the last decade. While fun and not too bad, none of them really tried to go beyond what the original did and in many ways they took steps backwards or dumbed down and simplified aspects.
This isn't a Firaxis phenomenon, it's the general industry approach and, in fairness, it's not new. On topic for this thread, X-COM: Terror from the Deep was a lot longer and tougher than Enemy Unknown, but it was very much the same game reskinned and didn't do anything to develop the game engine. Sequels have almost always had more to do with rewriting the existing code for higher-spec machines than with genuinely expanding on what went before, let alone improving on it.
Or look at the most recent Total War game (not counting the - ugh - mobile phone one), Shogun 2; compared with its two immediate predecessors it has reduced micromanagement, automatic teching (rather than needing to manage researchers and research buildings), heavily simplified tech trees, many fewer building types, fewer factions generally with fewer unique traits and units, less characterful, Diablo-style uplevelling instead of traditional TW character development, less decision-making in combat (most units have only one or two unlockable abilities, as opposed to formations you could research and that most units could use in Empire) and centralised provincial management in place of needing to garrison and manage individual settlements within a province. There's only one 'class' of unhappiness rather than two, and unlike still older TW games you don't need to manage growth in provinces in order to unlock new building techs or building 'slots', you just need to have enough money to upgrade.
Though whatever one's perspective on Civ V vs. Civ IV (or indeed Civ IV vs. Civ III, which would also be covered by the above generalisation), in that specific instance it's hard to claim that the game has been simplified or 'dumbed down' in any significant aspect compared with Civs I & II - gameplaywise the first two games will always be the classics, but they are much, much simpler mechanically and with many fewer strategic options.
Incidentally, the tip on the thread that the original X-Com games are on Steam prompted me to pick them up - wow has this game stood the test of time well. As quaintly pixellated as the graphics are, and as clunky as the interface is, it's every bit as engrossing as I remember (while a brief run with Master of Orion a while back persuaded me that in that case it might be for the best to leave it to nostalgia. No, as a 4x space game it still hasn't been surpassed, but that is one game where the interface really shows its age).
As for the new one, I'd probably rather they didn't market it as an X-COM game, especially when doing such bizarre things as turning Cyberdiscs into transforming spider robots, but if someone were to design a modern game inspired by X-COM, it looks pretty much what I'd like them to come up with.
There's the annoying Diablo uplevelling, the overdone super techs, and the spoonfeeding interface ("this is a wall, but in case you hadn't noticed, or didn't realise it's hard for enemies to shoot through walls, we've stuck a cover icon there for ease of reference" - and if Civ V is any guide, there won't be an option to disable this kind of unwanted "help"), all of which seem to be necessary evils in modern games (I was annoyed when the Civ series added Diablo-style promotions in either III or IV, though I've grown used to it), but I expect it's something I'll pick up and enjoy while thinking about it as something other than an X-COM game (like the various latter-day Apocalypse knock-offs a few years back - Aftershock and so forth). Probably when the price drops below $50 (not least because I can pick up a pack of the original 5 X-COM games - only the first of which I ever completed, and one of which - Enforcer - I'd never previously owned or wanted, but which is a good deal coming with the original three plus the entertaining, and no classic but nevertheless underrated Interceptor, and so giving me many hours of play even before I get onto the replay value).
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