If you'd like to see a particular game on Stadia, your best bet is to contact the developer/publisher via support email and/or social media like twitter/facebook. There's a lot that goes into bringing a game over to the Stadia library but a way you could really help the process is to make it known that this game is highly desired.
I would also recommend dropping the game request in the Stadia App via the feedback feature. Just navigate to the Settings page and scroll down to the Feedback menu item. This would also put it in front of some more eyes and show the Stadia team that there's an interest.
I have played Warframe on the PS4 and steam for many years and TBH i really dont think they will bring it to Stadia unless they can do cross play with it. The game in itself has lost ALOT of the player base and is dwindling more and more so i dont see them making that port, but i could be soooo wrong, also the new content they are dropping isnt getting very good reviews by the hard core player base from what ive read.