Or you could just play Xenonauts, if you’re looking for something like the original X-COM games.
Xenonauts is fully destructible 2D with somewhat craptastic performance, but it looks reasonably good, going for a somewhat realistic military sim sort of visuals.
It has a more elaborate air combat minigame than both the original X-COMs and the Firaxis re-imaginings. The key to which is to utilise the keyboard shortcuts for barrel rolling to a specific side - which I mention here because a surprisingly lot of people didn’t realise this is possible and consequently had a hard time of it.
Xenonauts also lets you deploy up to 12 troopers on ground missions. And base management is straight out of the original X-COMs.
The GUI in Xenonauts is perhaps a little bit too faithful to the originals, but still significantly updated and the only really serious flaw with it is the lack of a button to open and close doors - something that will cause misclicking.
In terms of combat mechanics, Xenonauts largely uses the exact same as the originals. The most significant change is the inclusion of weapon ranges. It should also be noted that the armoury in vanilla Xenonauts is more limited than the originals. In vanilla Xenonauts there are only 6 distinct weapon types. Researching and manufacturing new weapons simply gives you improved versions of one of these 6 types. Researched weapons are not functionally different from your initial selection of weapons. A laster carbine is simply a better assault rifle, and not a different type of weapon as such.
Xenonauts has a wealth of mods, including two very elaborate total overhauls, neither of which is in any way recommended for a first time playthrough, because they’re both hard as nails and come with more moving parts than you can be expected to deal with as a scrub.
That said, there are mods I would recommend for a first time playthrough. First and foremost I strongly recommend the Xenonauts Community Edition (XCE or just CE), which you can get by enabling beta updates in Steam. Not long after the game was released, the developer made the source code available to a select few individuals in the community, who have done an outstanding job fixing little bugs, tweaking the odd game mechanic, significantly improving the AI, and incorporating the best community-mader map packs. XCE is fully savegame compatible with vanilla, the most up-to-date way to play the game, fully compatible with just about every mod out there, and in the words of Chris the lead developer: “the best way to experience the game right now.”
In addition to XCE I would recommend using Fire In The Hole! It lets you breach UFO hulls, which in vanilla/XCE are indestructible (only indestructible objects in the game), it adds a few new UFO interior layouts (UFO interiors are not randomised in Xenonauts, though the tactical combat maps are randomised in the same way as XCOM2 by Firaxis - meaning there’s a general map layout with randomised elements injected in), and a couple of other little neat touches which, in my opinion, greatly enhances the play experience without significantly changing it.
There’s also a few graphical mods you might want to look into, but honestly… Xenonauts already does look reasonably good for a very low budget 2d indie, so the mods are nothing ground breaking.
Finally, though it is a long, long way off, Xenonauts 2 is now in the works and something akin to a design document is available as a handful of topics on the official boards. If you love X-COM, you should probably go read those. From the looks of things it’s going to be very different, yet very similar in all the right ways. I’m super super excited about it. Though, again, it’s a long way off.
EDIT: It might also be worth checking out Invisible, Inc. At least for me it scratched the same sort of itch that the Firaxis XCOM re-imaginings did, but honestly did it far better than either of those two games. Invisible, Inc is a turn based tactics/puzzle game with dystopian flavouring, where you infiltrate evil mega corporations, knock out their private security, hack their computers, and empty their safes. For a game with a drastically different premise than the Firaxis games, it actually plays a fair bit similar. But - in my opinion - does a better job than either of those. For example, in Invisible, Inc much of the pressure to get stuff done fast, is pressure you put on yourself by trying to go for bonus objectives. I can elaborate greatly if you guys are interested. It’s a very good game. But I think I’ve derailed enough for now.