What is the theoretical limit of how big Segwit blocks can become?

I believe currently the segwit blocksize limit is 4mb - but since segwit txns still have a foot-print in the 1mb-limited space, segwit blocks have a theoretical upper limit they can be.

What is that limit?

Although I suppose in the future there could be segwit-x transactions which do not have any usage in the 1mb space - thereby removing any ceiling on BTC blocksize. That is if the Core Devs allow it.