Sorry for the late response. I thought I had responded sooner, but I see that I have not.
The X-Com Files is also very difficult, so beware.
Hardmode X-Com, Final Mod Pack, and my Faithful mod are all fairly true to the original spirit of the game, varying in which directions and how much we stray from the original. Mine is in between, while Hardmode is more similar to the original game in spirit, and Final Mod Pack deviates further.
In fact, most megamods are quite difficult, usually significantly more difficult than the original game. If you're looking for something easier, I have four recommendations. These are in order from about the same difficulty as vanilla to easiest: UNEXCOM, Reaver's Faithful (on Beginner), Equal Terms, and Reaver's Harmony (on Beginner). Harmony deviates quite a bit further from the feel of the original game. I tried to stay true to what I saw as the design goal, but made many concessions in terms of gameplay for the sake of balance and gameplay freedom. But Harmony features an advanced difficulty scaling system, enabling you to choose a broader range of difficulty than perhaps any other single mod in the whole community.
The X-Com Files is very long, and it stays strongly within the universe of X-Com while straying very far in terms of the feel of the original. Story-wise, it's essentially the same game yet fleshed out much further. Gameplay-wise, it's like night and day. Final Mod Pack and The World of Terrifying Silence (TFTD) both play to the same idea: fleshing out the existing lore in new interpretations. And then X-Piratez and From the Apocalypse are both mods which fit within the vanilla storyline but in a different time period. From the Apocalypse is mostly canonical (based on X-Com: Apocalypse), while X-Piratez is purely fanon. X-Piratez is also a very difficult mod, and it is the largest and most popular mod in the entire community.
Final Mod Pack has been VERY extensively tested and should be generally bug-free. But Solarius Scorch manages it and is active, thus he will fix any bugs which do crop up.
The difference between Harmony and Faithful: Faithful is my original mod. A spark of inspiration gave me a huge list of major design changes all at once, so I ceased development of Faithful and re-branded the project as Harmony. In essence, Faithful is just Harmony but younger and less changed from the original. If you're worried about bugs, I would push you more toward Harmony than Faithful. But just the same, if you report to me any bugs with either, I will get them fixed--though not necessarily in a hurry. Fastest response is to message me on Discord, next fastest is to send a PM here on the forums. But allow me to advertise Harmony, my baby, to you:
- It doesn't really contain new aliens per se, but there are some unique models and recolors for certain cases. The alien lineup is more diverse, but not in the way that Area 51 is. Perhaps it is more lore-friendly in that respect.
- There are many new weapons added; most are based on fleshing out the original lineup, but several weapons are entirely new. This includes not only weapons for soldiers, but for tanks, aircraft, and even base defense.
- Manufacturing is massively fleshed out, with tons of room to make good money if you spend your resources wisely. Your engineers will never sit around with nothing to do. But there is no easy money, either. The manufacturing system is very well balanced, and making the best income relies on a combination of collecting alien loot as well as knowing what to convert it into for sale.
- Terrain diversity is slightly improved, but mostly vanilla. I plan to merge Community Map Pack into it one day, but it's not likely soon. You can run both Harmony and Community Map Pack at the same time, though some users have experienced bugs in the process. If you do, I recommend the basic version of CMP, and it is important that Harmony goes underneath CMP in the mod list!!
- Missions are a bit more diverse. In particular, there are more UFO types, and they generally have multiple internal design layouts. Missions have greater variance in which materials you are likely to receive upon completion, so if you learn where a material comes from, it becomes easier to farm it.
- In terms of holding onto the lore and spirit of the original, I took a hard sci-fi interpretation of existing canonical lore and changed as little as possible. There are a few cases where I tweaked original lore slightly. But mostly, I built it into a much more rigorous and sensible system that could be more believable as an actual alien invasion. I have stayed true to the original spirit of UFO mythology being generally true stories, but have steered away from the campy Green Space Ninja feel of the original.
- Harmony has some similarities with Area 51, perhaps in spirit. They aren't particularly similar in terms of gameplay or even modding strategy. Area 51 generally maintains exact vanilla while building on top of it, while both Faithful and Harmony slightly and sometimes significantly tweak the original in almost every aspect, while Faithful in particular adds very little new content, and most of Harmony's new content shows up in the late game to help keep the game fresh for longer. Area 51 seems to feel that the original game's balance was good, while I felt that the original game's balance was abysmal.
Thanks for your interest! I have tried to remain as honest as possible. If I succeed in steering you to try my mod, hopefully it'll be without disappointment! =)
And if you are disappointed, let me know and I may adjust the mod or at least update my statements regarding it.
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If you do give Hardmode X-Com a try, I suspect that the difficulty will be the only thing you won't appreciate. Beyond that, it sounds like essentially exactly what you're looking for. It's talked about less because it's older and hasn't been in active development for some time, but I've been chatting with Hellrazor, its author, recently. He may be coming back to re-kindle development of the mod. I've been feeding him some good ideas, and he seemed excited to discover he has a fanbase. I'm sure he'll appreciate any support, feedback, or whambulance you might have for him. XD
And no mod list is complete without mentioning 40k Mod, a total conversion set in the Warhammer 40k universe.