"Obliviate" genjutsu, if possible, would be awesome. We would be able to give anybody we want an NPC treatment (try convincing them, in case of failure wipe their memories and try again, repeat until success or running out of chakra. If success was not reached, just wipe memory of the encounter and disengage, try again after replenishing mana and preparing better arguments)
"Men in Black" seal also sounds really cool. But we will probably forget about it immediately after we develop it successfully. :lol
I think we already had the discussion about making genjutsu seals, and QM said it's not possible or extremely difficult, so let's reserve these ideas for the far future and continue our search of superpowers.
Well the MiB seal isn't a genjutsu in the "requires imagination" sense, it's just wiping short term memory and recently formed long term memory, so I think a seal should be able to do it.