Perhaps the most likely evolution for a breakthrough Google-free phone wannabe is not try to duplicate Google with a big app store, but to act as a secondary device, e.g., for keeping passwords, or 2FA information, or managing other things that a Google (or Apple) phone doesn't do well(*). Of course 2 SIMS is too expensive, but the SIM itself is a security risk. If necessary the secondary phone could VPN through the primary phones SIM. (*Personally I don't keep passwords or 2FA on my main phone because (1) I doubt the security, and (2) in case the phone is stolen.)
More generally - suppose a company wants a secure device for scanning, measuring, tracking, communicating - a cheap cell phone form factor with an ultra dry OS the allows a single app to run without a lot of background junk - maybe in near realtime - that could be marketable.