This anime game. This would raise the number of available five-star banner characters to nine per update, providing more options to pull for in each version. It's likely a necessary change when looking at the ever-expanding roster – characters like Eula haven't been on a banner since late 2021 because there just isn't enough space.
Genshin Impact version 3.4, and Shenhe is confirmed to be coming in the second half of version 3.5.
However, Eula has been missing since November 2021, and there's been no official mention of her banner at all recently. She won't even be on a banner during the Mondstadt events next update, which is bizarre, given the fact that four characters who have nothing to do with Mondstadt – Cyno – will be available.
When HoYoverse begins adding new characters to pull for from Fontaine, older options will start to get fewer and fewer reruns. Each update is currently about six weeks long, so the banners can be split into two weeks each with a third phase.
Adding a third banner phase just makes sense, and this leak from Team China suggests that this will happen sometime after version 4.0 goes live. Team China's info has been translated and put in the Reddit thread linked above. It seems as though this change will come before version 4.3, but after we've already entered Fontaine.
Version 4.0 is still a ways away, though, so it's best to focus on pulling for Hu Tao and Genshin Impact version 3.5 goes live next week.