under water LEVEL, with a underwater ( deeep underwater going down levels that are made of going down a a cave like hallway that is a part of the level itself, as the player goes down. but forget that i said all that and lets start here firs: a underwater area where the player after swiming a bit underwater and finding a underwater cave that then leads to a deeper part of the cave thats not underwater, but a underwater forest, or a part of the cave that has a forest in it. a cave forest.
im not to smart right now btw, when its my days off i may come off stupid sometimes so sorry about that.
i also know a thing or two about how the engine works. ill have a room that will slowly fill up with water and turok must do something to make it stop, and to escape. useing a item he must pick up and place into something to stop the water flow.
Doesn't sound stupid at all. Interesting drain the water before you drown. Im liking these water Ideas. Underwater forest hmm sounds crazy but crazy can fit into Turok easily. Imagining this it made me think of a big 4 way cave. Each exit/entrance takes you to a new area. But first you must stop the cave from flooding and Turok from drowning. Just imagining it though.
Adding to my water level ideas*
How about and underwater temple.
While your underwater there can be spots where you have to go up and into maybe caves that you can only enter from underwater sort of like level 1 where you get the pistol in the start secret.
Its kind of making me think of "Night Dive" references since it will be underwater 
Maybe Islands can be spread out for ammo
I am thinking about an area that has been flooded that didn't use to be flooded it would be ancient and with Turok that can be easy to achieve.
Adon you might not be a mapper but you gots good ideas going really.