I know with my YouTube name, it's just a nickname that I earned a few years ago, and I stuck the 64 on the end since the Nintendo 64 was my first console. My channel's recently become more a Pokemon-centered channel, but I've done other, non-Pokemon related content. So I would recommend picking something that sort of hints at the series, but isn't overly obvious. Looking at some of the PokeTubers, MunchingOrange, TamashiiHiroka, TheJWittz, none of these really have names that are really screaming Pokemon. The only ones I know that have blatant Pokemon-related names are channels such as PokemonReorchestrated, TheNationalDex, and that's it really. So I would pick something that you would think is easily remembered, and not too complicated sounding.