Hope for a Hybrid game. That indeed could be so fun! Any advance or mod was able to simulate it?
My
X-Com Files pretty much has it, on a limited scale. It contains a story arc where 1999 X-Com has to deal with an early emergence of T'leth agents. It features some underwater missions, both in wilderness and industrial complexes.
But it doesn't include real USOs, I mean ones that travel around the globe and do missions in regions. It is site missions only.
Also, the mod is playable, but very WIP.