Russia's tech giant Rostec decided to spice up and add the intrigue to the upcoming MAKS-2021 and announced (in Russian) that UAC (United Aircraft Corporation) will present at this salon an absolutely new combat aircraft. They even started the countdown to this presentation and stated that the 30-second video gives some clues about this mysterious aircraft. Judging by the name of the web-site by UAC--and it is titled Checkmate--it is designed not only for Russia's VKS but for export too. From a shadow of this aircraft on the water one can conclude that it is something of MiG-35--SU-57 variety, but I also saw two booms in the back of this shadow. Could it be 21st century STOVL related heir to Yak-141? Possible. Russia never denied that she works on such an aircraft.
Here is MAKS-2021 official promo but it doesn't disclose anything re: this brand new aircraft.