I think now that Umbra Forma exists, I dont think every warframe will receive the Umbra treatment because ANY warframe or weapon can now have access to the Umbra Polarity.
Youd be wasting it on a none prime, but still anything now can become "Umbral"
I still think well get more Umbra frames. I just think its going to be the frames that first shipped with the game and possibly in the same release order as the Primes (Frost being #2).
To me this makes the most sense, as those frames deserve to recieve that treatment. But because of the Forma.....any Warframe can be an "Umbra"
Im leading with that because I dont see DE giving us a "Custom Warframe" per say. But what they can do is tie the Umbra Formas and the Infested room together.
Place an Umbral forma into X warframe, and take it to the chair. The chair is basically a mod table where you can change X stat on a warframe: Health, Armor, Energy etc. Each Warframe is now customizable to how you want it, this will have to be balanced, but its something that would be very interesting to have as each frames playstyle will change.