It's your job as DM to facilitate the players to build the character they want to use.
Nope. Your job is to promote ROLE PLAY.
Back to my survivalist example, I would't allow create food/water in this fictional worlds. Cuz both spells would END with all of the need for survival mechanics and all intrigues about "who owns the water sources?" This doesn't mean that the player can't for eg, create a wizard or druid which his goal is to learn a way to make infertile sands fertile terrain and to discover how to transmute, create and purify water and solve the starvation problem, but I as a DM would only allow him to succeed on late stages of the campaign, so we can end a campaign with the player character being the guy who solved a great problem plaguing the world.
If the PC wanna play as a dragon born in a world without dragon born, is up to the DM to not allow it.
Multiclass is often used to bypass character weakness or create pun pun builds
What is the greatest weakness of mages on 2e? Very low survivability, worthless without spell slots or in anti magical field and very innefective against high MR mobs.
What is the weakness of Fighter/Mages on 2e? None.