Threema seems to solve a problem signal has that is it does'nt need a phone number to open account . But i haven't used any of them so can't say . (If anyone wanna know i use telgram foss which is a debloated fork of the original client)
That's not really a problem. The biggest problem Signal has is people not caring about privacy enough to use another messaging app.
If Signal dropped the phone number requirement they'd get a handful more users. If people started to care about privacy they'd get millions of new users.