I think it's unlikely to see a NijiEN > HololiveEN move among the girls. One of the guys, maybe? That said, the obvious to move to watch for is one of the girls going from NijiEN to Vshoujo. Considering not-Pikamee's very high CCV right now (we probably need to start asking about the truth of CCV counts on Twitch and YT right now), I could very much seen one of the girls make the switch. Depending on their friend network.
From a purely business stand point, depending on what they're actually making in Nijisanji at the moment, it might actually be dumb they haven't made the switch.