This is a problem I've been having in general so far as a Game Master; some character classes seem to be designed not for party play, but to be "loners" and shine only in specific situations. Some players like them, some pick them, I just don't know how to "Master" them.
Some examples of my history with said classes:
D&D 5th Edition. A player creates a rogue player; he loves stealth scenes, assassination, etc. He notices he's become the "official trap disarmer" of the party, and every now and then he gets to shine in stealth sections, however since half the party wears heavy armors they usually crash the fun for him or he has to go alone, fails some die rolls, and is placed in a very dangerous and life threatening situation that doesn't gets any better. He soons starts hating to be a Rogue because there's not enough chances to be sneaky and cut throats, and he becomes the "skill maid" and trap-damage-eating guy.
Pokemon Tabletop United: A player wants to become "too specific" as well and builds a character of the Rider class, so he can Ride on Dragon Pokemon. Tho it fits the game world (since we're playing in a Dynasty Warriors x Pokemon setting), he wants to use his very specific rider combat class every single session, however that also means the prep increases and the things become stale because every single encounter I have to think of another flying rider or he becomes too over powered since the rest of the guys fight in land. Also, he wants only Dragons on his team, so at low levels he has nothing to ride since he doesn't wants land rides, which has made using his whole "Dragon Rider" class very stressful, even if it fits the game world.
Anima: Beyond Fantasy: A player creates a scholar, he wants to play the role of the nerd that follows the group of adventurers and writes down everything about their travels while aiding them with her wit and brains. The game is high fantasy action, and what she does every turn is just run, hide, and spend real life hours thinking on what to do since her build means even a rock between her eyes wil lmake her explode in a bloody mess. She still insists to be the "puzzle solver" and "mystery master", so I have to think on puzzles just for her every single game...
We've used the Same Page tool, and as a group we have fun and we really like to play together, however it doesn't stop them from wanting characters that are too specific and require extra planning and solo scenes for them to really shine as the others do, which turns the roleplay into boring because during those scenes, they either become stuck, or want more of that "main character "feel, since there's no way they can really aid the party without getting bored, and sadly, RAW support those characters and I can't tell them "no" because all of them fit my game worlds...