X-COM is much more suited to an isometric grid than tessellated hexes. The main advantage of hexes is that the increased number of contacting spots allows for more advanced formations and melee engagements (movement can be disregarded, as most square grid games have a 150% cost for diagonal movement, which is close enough to the actual factor of sqrt(2) while leaving it easy to calculate, as well as offering a total of eight squares to move to instead of six hexes). This matters most in games where conflict takes place one or two hexes apart, like Wesnoth or Civ, but it would be wasted on X-COM.
Square grid combat plays into X-COM's strengths: use of line-of-sight and dynamic cover. The ready availability of a Cartesian grid makes it easy for the player to visualize their line of fire and how it might intersect with intermediary objects. I'll be disappointed if they switch to hexes just because it's what their last game used.
I also think that hiding the grid from the player would hurt their ability to think tactically. A button to do so would be acceptable, but I don't see why anyone would use it.
Hexes are great because they're the best way to map the world
While hexes would likely work well for modeling the world, I don't know why the geoscape would need to be divided up into a grid, unless they were making a mechanic of it.
Fallout is a rare game that pulled it off so brilliantly that nobody realized it was an ugly hex game.
While I love Fallout, I don't think that it's hex-based grid was at all suited to the environment, architecture, or combat.
I'm going to remain cautiously optimistic. "We're all big fans of the original" has become the industry code for "so we're turning it into an FPS", and there has been a general trend in the industry for reducing depth. If they want my money, they should implement Mephansteras' weapon suggestions, rebalance psionics and UFO encounter rates, and in general produce X-COM: More Content and Less Bugs.