Then there's a more appropriate one that wouldn't work at all. Rome: Total War or any Civ game.
There was an official Civilization 2 mod about X-COM
Day of the Tentacle:
You've got to use the porta potty time machine to send agents into the future (TFTD) or past (Piratez??) so you can play three games at once, in which timeline continuity is thrown out the window and changes you make in the past apply in the future at the same time relative to the way the player experiences them.
This one would greatly benefit if Geoscape can switch map and date/time on the fly. Even without that the mod would be workable, with missions to past and future launched from today. The Chron-o-John would limit squad size to 3 or maybe even 1. There can be researches to improve the time machine and missions to capture bigger diamonds.
The biggest problem is the plot, which is effectively written in research items and mission briefings. The research tree has to allow significant freedom to the player (per the spirit of DotT), yet keep the player jumping between timelines to add variety.
Plus, there's a question: if you can travel in time, can you save your lost operatives? And a bigger question: what will be the overall tone of the game — how bloody will it be? It's nearly impossible to keep everybody alive in X-COM (if you count the civilians), but there were no deaths in LucasArts adventures.
Let's say, the game starts with hordes of mutant evil tentacles appearing out of nowhere all over the globe, causing havoc. They sent Ethereals and Governor of Mars packing by the spring of 1999, solving your biggest problem, and becoming the major problem themselves.
At first you only react to signals of savage tentacles. Eventually you capture a moustached leader, one of many sent from the future (late 22nd century) to guide them. You find out that future tentacles are busy installing Sludge-o-Matic machines all over the world in the distant past (late 18th century) and the mutagen slowly transforms common cephalopods.
To prevent humans from interfering with their plan, future tentacles covered the last millennium with some neutralizing field that only allows time travel within a narrow tunnel, with way stations in 1999. They don't understand the time travel technology very well, thus they can't control where the way station is going to be, hence the need to guard the way stations with mobs of not-fully-mutated savage tentacles.
You can track down mutagen sources in the past, storm them and neutralize them, but that would create a problem. No mutagen — no tentacles in the present — space aliens didn't leave. If you figure how to scare them off, there'd be a paradox: no mutagen — no tentacles in the present — no reason to keep X-COM around that long — X-COM has been disbanded long ago — tentacles are back. Some researcher may claim you've already went that route and are now back to square 1. So, you need to
capture tentacle specialists in the past, where they are few, easy to find and aren't well defended;
extract knowledge of the future from them — locations of poorly-defended information and valuable equipment and materials;
use this knowledge for stealthy raids;
maybe attack some places guarded by tentacles to sabotage something in the present and past to compromise future defences;
there can also be cases when a prisoner lures you into a trap;
rinse and repeat until you can build your own time machine that can dig its own tunnels, all the while suppressing tentacle mobs, who get stronger, smarter and better armed with time.
You will find there are people, who know tentacles well: Bernard, Hoagie, Laverne and Edisons family. They've been captured, and you need to send rescue missions after them. Debriefing them reveals important pieces of information: you need Dr. Fred to build the ultimate time machine, Bernard knows where he hid the blueprints, Laverne picked up a lot of info about tentacle biology while she was their prisoner, Ed and Edna observed tentacles for years and are also needed for Fred's peace of mind. Can't think of use for Hoagie. Ah! He's so dirty, he's immune to the tentacles' toxins.
When all is said and done, you discover that the thing that scared the aliens away is the Slugde-o-Matic mutagen — it makes the servitor races rebellious. They didn't leave in fear, they started a civil war. So, for continued existence of independent Earth X-COM can't just cancel this tentacle affair. It has to take the place of future tentacles, take over the mutagen production, and integrate the sentient tentacles into the human society. And to come up with a plausible cover story.
How about that for a mod?
The biggest gimmick of DotT was immediate changes in the present and future, when something was done in the past. The "immediate" has to become "when the space-time ripples reach our year, usually no more than a month". I know the events can be triggered by finished research, but can they be triggered by successful missions?
