I almost always agree that a TAS should be the hardest difficulty possible, but in the case of Doom, the games often have an exceedingly unfair highest difficulty. According to the Skill Level page from Doom Wikia:
http://doom.wikia.com/wiki/Skill_level#Doom_64_skill_levels
Doom 64 has 4 difficulty levels, with
Watch me die! being hardest. Apparently, difficulty only changes the amount of monsters. Of course, here at TASvideos, we need to know more. In some Doom games, the player takes extremely higher damage, and enemies are much tougher and faster. If Doom 64 only adjusts the spawns, I'd say the TAS should be on Watch me die!... unless it's an unreasonably high amount of monsters (but Hyper Princess Pitch made it work!).
And finally... don't be too sad if it doesn't get published! This is a great framework for what could happen with some collaboration. What ends up being published doesn't need to be 101% perfect, it just needs some cleaning up. :]