From what I've discussed so far, the general consensus seems to be that such a hybrid could take 2 forms:
A) Serial - after completing Cydonia an finishing UFO the game continues but now in the TFTD storyline.
B) Parallel - both UFO/TFTD take place at the same time.
Both options have pro/cons but quite honestly I would prefer option B) since there isn't much difference between A) and playing both games separately. With option B) it would also be possible to either have the hybrid on a UFO or a TFTD setting (GUI, etc.) but I would also prefer to keep it based on UFO.
I've thought about this myself, without going too much into my own expanded idea for a game.
You would have to have both Cydonia and T'Leth destroyed to win, but it would be like the orginal Fallout, where you can do the victory requirements in either order. Fuflfilling either one will reduce alien missions from that group but the other brain takes over both groups. I would have T'Leth rise sooner than the Martian end game plan needs to destroy X-com. In other words, you might only have a year or 2 before game over if T'Leth isn't stopped sooner rather than later.
The 2 alien groups would play differently. I would have the sea aliens be on a harder level of difficulty, but fewer missions. The space aliens easier, but 3x times the number of missions.
Basically, the Martians can win without T'Leth, but it will much easier if they get T'Leth to rise. Because of that, the space invaders would get a new mission complete with its own ufopedia entry, Rehab (the space aliens are aiding thier sea brothers in restoring T'Leth). If too many rehab mission occur, T'Leth rises, game over. If you take out T'Leth first, well the eco damage wrecks the world economy and your maintence fees and funding go in the wrong direction.
World/Regions/Countries
Countries
Since TFTD introduces several new countries it will be necessary to add those. The easiest solution would be to check to see if there's a corresponding country in UFO, and if not, add a new country in UFO to match it and taking into account the political situation in 1999 or whatever date is best.
United States - USA
People's Republic of Alaska - USA
Eurasia - Russia
Euro Syndicate - consists of UK, France, Germany, Italy and Spain on UFO.
Asian Coalition - India
Scandinavia - add Norway
Neo-Japan - Japan
New Mexico Cooperative- add Mexico
Brazilian Union - Brasil
Australasia - Australia
Egyptian Cartel - Egypt
African Corporation - South Africa the most likely choice
Icelandic Union - add Iceland
Arabian Block - add Saudi Arabia
Free China - add Taiwan
Federated Korea - add Korea
This would mean to add 6 new countries to UFO. The issue afterwards would be to split TFTD's country zones (where UFO/USO activity is calculated) between UFO's countries.
I don't know if its possible, but what if when the sea aliens start attacking, 6 new countries are brought in to do additional funding?
If the aliens are opening up a second front, it would make sense if more countries are brought into the fold.
Damage Modifiers
Both are essentially the same, so they should be stuck together in the same system and rename them as required. Lobstermen would still be vulnerable to both melee and stun weapons, whether it's on land or sea, etc.
I know it would be a pain to code, but I would like it if the different types of damage where kept seperate. Granted, someone would have to decide how much plasma and laser does to a lobsterman and vice versa, but it would expand the game.
I would also like it if lasers and plasma don't work underwater.
The space aliens should eventually switch to sonic weapons.
Sonic craft weapons shouldn't work on airplanes since high alitudes have less atmosphere which is required for sound waves to travel.
Crafts and Interceptions
There are 3 main issues here:
1) UFOs being shot down over water
2) USOs being shot down over land
3) USOs being intercepted on the air
The issue with 1) is what happens to the UFO and if it is possible to send a sub to recover it. However, land aliens should not be supposed to survive on water, so either there would be no combat involved (or even no mission created if we consider that the UFO would simply be too wrecked for any salvage) or the land aliens would be replaced with aquatic ones.
With 2) it would be possible since TFTD already allows for land missions, so the sea aliens could be present on USO recovery missions on land.
Now 3) would present the biggest challenge but it would also be an incentive to have an interceptor ready to deal with any USO that surfaces during an interception. Or it could also be possible to mount aerial weapons on the Barracudas since they can fly.
Finally, there's another option - UFOs being able to submerge (to escape Interceptor attacks) and be intercepted underwater by Barracudas. This requires solving out the issue on 1) though (along with all the technical changes to the engine).
Eleruin isn't suppose to survive contact with water, so no space aliens under water.
I would allow land bases to deal with sea terror attacks, shipping lanes.
Bases/Facilities/Soldiers/Equipment
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It might also be possible to introduce a 'hybrid' base that would be placed on the coast, that would allow to have both land and sea facilities present but this would have to be carefully considered, since it would be the most logical choice for the player, and most likely the other 2 bases would not be used.
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Specific facilities (either can only be built on land or sea)
* Radars/Sonars
* Hangar/Sub Pen
* Hyperwave Decoder/Transmission Resolver
* Psionic/MC Lab
* Defenses
Soldiers/Equipment
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* Gauss and Sonic technologies are another matter. You should be able to use both on land but at the same time you ran the risk that players will keep them instead of upgrading to plasma. However, this can be solved by having completed research on plasma weapons as requirement for developing both weapon types.
First, sea bases have to able to be a hybrid as space aliends can land attack them. Remeber, the water bases are called "Floating Bases," which means part of them is above the water. For that reason, I would allow them to be able to build all land facilties, but not the otherway around unless they are made on or near the coast.
As for not developing plasma weapons, unlikely. Heavy Plasma has 35 shots to Sonic Cannons 10. H Plasma has auto fire unlike Sonic. H Plasma has a snap shot of 30%, sonic 50%. Granted, Sonic is 5% more accurate but that's not much of a trade off. Sonic Cannon has an average of 130 (+/- 50%), which translates to a damage range of 65-195. Plasma can do 0-230, average of 115. Yes, sonic has a better average but when your facing very tough opponents, plasma will more likely get through.
For the record, plasma should not work under water.
Research
This is probably the section which will involve the most changes. Essentially the idea would be the same described in the plotline section: you need some TFTD research completed to advance your research in UFO and vice-versa.
Some ideas:
* Plasma Weapons required to develop Gauss technology
* Hyperwave Decoder + Live Sea Alien Navigator required to develop Transmission Resolver
* MC Lab + Live Land Psionic Alien to develop Psionic Labs
With this sort of logic the Research Tree would become more complicated and harder to complete but it would be more integrated. You could only advance your land research if you conduct some sea research and vice-versa. It could also set an interesting decision for the player, since to focus in one would mean to be lacking on the other.
I wouldn't intergrate the research except I might change turn matter control into mind control
