Guiding player character creation
It’s party time.

When overseeing character creation, remember that the more obscure the player’s source material, the more work it is for the GM.
Balance flavor with demands. It’s okay to say no to the hengeyokai coconut crab gunslinger if you’re running a norse-themed campaign. But compromise when you can.
If the chosen class or race does not yet exist in your campaign world, could it? Should it? Can it just be renamed? If your setting has a European flavor but players are still choosing Japanese themes, a samurai could easily be a noble knight, or a ninja could be a shadow assassin.

It’s okay to enforce alignment restrictions. If you’re running a heroic campaign, you don’t need to allow evil characters. But if it’s a more mercenary or intrigue-themed story, then anything goes.
It’s also okay to enforce some naming conventions. If you’re running a reasonably serious game, you don’t need the constant distraction of bad pun names to destroy the atmosphere.

If characters favor exotic weapons or equipment, make sure players understand availability within the world and accept that if their characters lose their favored weapons it may be tough to find a new one in their current region.

When integrating player ideas with your existing religious pantheon, you may have problem players for whom nothing is quite right, insisting on creating their own religions. (In my primary campaign world, I offered a choice of 64 deities, which still proved insufficient for some.) Create saints of existing gods or offshoots of existing religions. The players may still complain about the lack of temples that have failed to miraculously spring up to herald the birth of their adventurer, but they can use existing related ones.

Be aware of how character roles and specializations can influence the game. Loner hunters will still have to work with the group. Choices like investigator or bounty hunter can heavily influence the campaign style, otherwise you risk player frustration. But you don’t need to remove all the dungeon-delving and dragon-slaying in favor of whodunnits for Holmes and Poirot. Players cannot reasonably expect that. But you cannot expect all players to be reasonable.

It’s okay to invest more in players who are the most willing to work with the world offered them. As for the others, the more demands they make initially, the less likely they are to last through the campaign. Be prepared for character switches when their initial creation proves less useful in different situations. Be prepared for them to quit outright. Don’t waste too much effort on trying to shoehorn the whole campaign into their rigid vision. Release them to go and enjoy that solo video game experience that they’re really looking for.