I've not played UFO 2000 for a while, I assume Lua is for scripting events within the map? There has been talk of adding a scripting language to OXC in the past.
To tell the truth, I have played UFO2000 once or twice but it is not the case. Lua files describe how to make a battlefield of tiles (stored in terrain folder of the game), and of map files (maps folder of the game and additional folders with used-made files, shipped within the archive of a given terrain). Besides, there is a function called MapGenerator which tells the game how to link map files in order to make a correct battlefield.
There are many maps of this kind ready for using with UFO2000. If OXC could import them, it would save additional effort of map makers. In fact, at least some of the maps are so fine that people do make conversions to use them with original games.
https://www.ufopaedia.org/index.php?title=Custom_Maps_%28UFO2000%29 uncovers some further technical details. And the map depot is here:
https://area51.xcomufo.com/depot2.htm.
Why would you continue after you beat the game? Where would it then end? I suppose if you just wanted to keep playing, but without a mission and full research, seems little point, you might as well just use Free Battle.
Free Battle is not a campaign. As for me, the charm of XCom games has always lain in strategy, not only in tactical game. This is why tactical-only games (like the Gollops' Laser Squad Nemesis) have never been so popular.
If "the 10th wish" fulfilled, the player would need to accomplish both the EU aim (Cydonia) and TFTD aim (T'leth) to finish the game. Just like WeOwnTheNight wrote, it would be one long campaign. And still not the same as playing the two games one-after-one.
The main difference would be that you could start the second part (TFTD) with trained soldiers and with advanced weapons (from UFO:EU) for land missions (now inaccessible in TFTD). Even now XComUtil (and, as I can remember, also some other game tools) can transfer existing soldiers from UFO into TFTD, so there is such a need among players. Besides, even now there is a division into universal weapons and underwater (only) weapons in TFTD; in this scenario there would also be land-only weapons as well.
In order to balance the game, you might have a little tougher underwater creatures just from the start of the second part, as your soldiers would be trained. On the other side, you would still have weak weapons on underwater missions. Or maybe some of the trained soldiers should not be part of underwater missions (because of medical contraindications?) - now you can have extra fighters but with low psi, in the proposed scenario you would have extra land fighters with low underwater skills etc. Result: longer campaigns, twice as many weapons as in the original game, but many usable in one environment only, more player decisions (who should be sent to a land mission and who to an underwater mission, what weapon should be loaded into the flying sub, etc.), just more interesting game.
Running both simultaneously has also been discussed, but that would be a lot of work - especially if you're wanting to double up bases. I would expect research tree changes would be rather trivial though
I do not say it would be simple but I think it would be a really interesting and expected option. Think positively. I wonder if it really needed making larger bases: TFTD bases are under water, while UFO bases are on land, so hangars and sub pens side-by side? On land? If there is a need to have larger bases, that would be little related to simultaneous game. Or perhaps a special base type in ports, with all 4 levels used, 2 lower for sub facilities, and 2 upper for air facilities. Just an idea.
Is the 8-base limit so hard to be removed from the game? If it is really, so OK, 8 bases totally (including some land, and some underwater, say 4 + 4). It should be enough to win the game. Adding TFTD weapons, subs, aliens to UFO:EU should not be very hard after having TFTD reimplementation finished, and it is quite possible that it would need mainly changes in the ruleset.
Let's wait until reimplementation of TFTD is finished
![Smiley :)](https://openxcom.org/forum/Smileys/default/smiley.gif)
Then we will be talking about details of a possible fusion.