does someone knows the next update's main girl?
We don't know, but my money is on either Eri or Yui.
Logic is, new girl probably getting content at the same time as the girl she's connected to. Onori already has Aiko so probably not her (and I think I heard that the next Onori update needs Aiko progression first, so even more unlikely as it would mean the next update would need to add content for 3 girls if that's the case).
Suzuki just had an update so probably not her unless the dev wants to do her twice in a row.
This leaves Eri and Yui. Out of the 2 I think the more likely one is Eri because her side girl will probably be Mien-Mien, the girl that is probably a mad scientist and leads the school club Eri is a part of.
Could be Yui, but unless I missed something I don't think they have even hinted at a side-character for Yui (with Aiko at least Onori was introduced as having more friends in the past), would be kind of cold turkey compared to Mien-Mien.
There is the possibility there will be no "main girl" and the dev could instead do a "side girls only" update with Aiko and the new girl, but I see that as very unlikely
unless the dev wants to update Aiko before making Onori's new update to build hype for her return.