Also just for my own curiosity - are you planning on making the kopru alien invaders rather than Undersea dwellers?
I have a solo player doing a survival game on the Isle of Dread right now and the idea you just gave me of mashing up Isle of Dread with Expedition to the Barrier Peaks and having the kopru temple be a crashed spaceship might be too good for me to ignore...
This post by @Jer (Thanks!) inspired me to take a look at some old module that I am familiar with and i have made some changes my campaign.
I've just decided to mash up Valley of the Hutaaka (Night's Dark Terror) with the "Neanderthals" from Isle of Dread. This will upend some Mystara lore.
The Hutaaka (fox-people) were a subservient culture to Naraj's Rakasta (Lion-people) followers. The Hutaaka are descendents of the the culture that created the Lost Rakasta Shrine, along with the currently cursed Temple of Kortoeba. Their sacred duty is to welcome the return of Naraj El-Najar, though they really don't know why. They just know that it's important. The double-super-secret-hidden valley of the Rakasta Shrine can be accessed through a long tunnel found in the Pit of Kortoeba; AKA the Guardian of Naraj. The current Pflarr religion of the Hutaaka is unknown to the Rakasta.
The Traldar will need to be renamed, but will have the same ability and stats from the 5e conversion that I have (available for free at Pandius site). They are another group of Isle descendents that are related to the Tanroans.
The group of Traldar met on the outskirts of Hutaaka Valley are escapees from the Valley and just found these futuristic caves three months ago. They refuse to return to the valley and have no love for Animal-human hybrids. A demonstrably useful magic item could be traded for a rifle (both bosses will be vulnerable to radiant damage.)
What to rename the Traldar?