I think it would be an interesting occurrence if sometimes slaying a titan opened up a portal, either where they were felled or, hopefully, even on them. These portals could go to old Oortian palaces/fortresses or vaults with unique or rare loot or just be amazingly visual if nothing else.
However I would be even more in favor of titans with portals on them in some way, such as on a head crest or on their shoulders that you can go through only while they are alive and find the locations from above. That would require there being another exit from the vault or fort/palace, of course. Wouldn’t want to be stuck in a world if someone else kills the titan, would you?