In all fairness, I think its just fans commenting off the top of their head as the next natural progression.
Terror From the Deep added very little value to the original game. People don't really talk about TFTD.
Any XCom fan that has played TFTD will know that it is largely a lazy sequel to generate money. If you look at the skill trees for example, they are a clone of the original game. Most fans won't even remember the names of the weapons and just remember - oh yes, its the laser equivalent or the plasma equivalent.
The only 2 games that were really worth their salt were UFO: Enemy Unknown and XCom: Apocolypse.
I do think that there is scope for the development team to create their own ideas for underwater missions - but there is little to get from TFTD.
However, it is good to get inspiration from other games. For example, in Apocolypse, enemies had energy shields which required a change in strategy. If you shot at an enemy with an energy shield repeatedly until it failed and then killed the alien, there would be no energy shield research because it was broken. However, if you gas grenaded everything onscreen, you'd capture at least one alien with a working energy shield which you could then start the energy shield research tree. Absolutely fantastic element of the game which required a change in thinking.
Kalten~!!!