Zardnaar
Legend
Player fiat, really. As DM I don't dictate what class compliment the players choose to have.
Personally I wouldn't want to be in a game where the DM says 'No, you have to play <insert class> because the group needs it' or 'No, you can't play <insert class> because another player is already running one.'
I more put warning labels up. During session 0.
" Play what you want but think about a somewhat balanced party. Don't deliberately make a bad character, you don't need to power game but you can't suck either".
I'll be tweaking that to rule out the following.
Four Elements Monk
Beastmaster Ranger
Alchemist Artificer
Berserker Barbarian.