I'm calling it now. Baizhu will either be the Qiqi or the Yumeglorp Glorp to Rimegrass.
Given that Mizuki and Qiqi were once deemed useless alongside the other Stellar characters pre-buffs, I have high hopes that Baizhu will be the premier support or DPS for Rimegrass. I expect his kit to be reworked, similar to the two I've mentioned.
The main problem I can see is that he scales off of HP. If we're going to take into account how most Dendro-based reactions go, Rimegrass would most likely be based on EM. But one thing I can think of to counteract this is if his passives would be reworked to be similar to Nilou's: "For every 1,000 HP Baizhu has past 30,000 HP...." and so on.
If Hoyo were to go the Mizuki route, here's my reasoning as to why that is: Mizuki and Baizhu have parallels in how they were treated. For one, Mizuki and Baizhu both have 4-star alternatives in the same niche as them (For Mizuki it was Ifa, for Baizhu it's Yaoyao). In fact, the 4-stars were seen as highly capable sidegrades instead of being used. Both of them had supportive capabilities, sustain, and mediocre cons. And they can both be drivers in their respective niches, but they aren't really the best. Hell, they're both catalyst users too if we're going there.
So if Hoyo were to go the Baizhu route, they'd most likely do something along the lines of a Baizhu on-field playstyle - where his slower Dendro application will actually be beneficial to the reaction.
If Hoyo were to go the Qiqi route, there are some things we need to consider. For one, will Rimegrass be a Stellar reaction or will it be a standalone reaction? I can see Hoyo making it go either way. If it is, then Qiqi might still be the sustain for it. If it isn't, then Baizhu buffs are more than certain. Or perhaps Hoyo might no longer be buffing/adjusting Qiqi for Rimegrass since they might incentivize you to pull on another buffed character's chronicled wish banner.
You might be thinking: "But we'll be having shielders and there's also Diona who fills the same role as Baizhu (shielder and a healer)". While that is technically be true, Baizhu may still prove to be superior in terms of sustain capabilities for Rimegrass, just like Qiqi is to Stellar reactions. And, similar again to Qiqi, they might not provide that many significant buffs compared to Baizhu. I do think the Diona equivalent of Rimegrass would be Yaoyao though.
TL;DR
In short, things might be looking up for us Baizhu mains. He shares parallels with the aforementioned characters. He'll either be an on-field DPS, an on-field driver, or the 5-star sustain/support for Rimegrass. Only time will tell if any of what I said will come to fruition. Also, I doubt he'll be a sub-DPS, as he has no sub-DPS capabilities to speak of and work off of.