Here's the gist:
Your current Warframe changes the UI to a fancy theme and background matching that Warframe.
[in the mannerism of Owen Wilson] Wow!
Maybe Ember has a smoldering flame background, burning at the borders. 🔥
Maybe Frost has snowflakes falling in the void. 🌨️
Maybe Oberon gets little twinkling fairy orbs flying about. 🎇
Very cool! Such immersion!
The ability to change the UI is neat, but currently also limited for an uncompelling monetization, essentially hidden away as a background feature.
I think that if the UI had a feature similar to the manner in which Warframe animations work (unique content locked to the Warframe, that enriches the identity of the Warframe in question, available for all who own it to enjoy while using that Warframe, and is able to be purchased to be used at all times), that it could become a more valuable and immerse feature. And the ability for the UI options to feature themselves through normal gameplay would serve as effective advertising for that vector of monetization.
I don't know the exact manner in which the background is rendered (is it just a static image being manipulated?), but perhaps assets for various in-game effects that already exist could be re-purposed in conjunction with simple art and palette swaps to, I presume without too much difficulty, create such immersion.