Following the Ubuntu 21.04 (Hirsute Hippo) release, there will be Ubuntu 21.10, which has been dubbed by Canonical as the “Impish Indri”.
Canonical also published the release schedule, suggesting that the development on Ubuntu 21.10 will kick off on Thursday, April 29th, 2021, with the toolchain upload. Of course, the development will be based on the current release, in this case Ubuntu 21.04, and the release schedule also suggests that the Beta version will arrive for public testing on September 23rd, while the final release is slated for October 14th, 2021.