My group (mainly one person in particular in it) really want to multi-class but not at that deficit that comes with that, spreading butter too thin. I gave them the option that perhaps there is a way in the game world to achieve two simultaneous levels (a gestalt level) for x amount of time, but that each consecutive one would be more difficult in some way than the first.
And this power could come from an artifact, a powerful magic user, an experience (traumatic or otherwise) the characters go through or an extraplanar / divine or infernal experience.
I was thinking about making it a mix between hard choices and hard encounters - splitting up difficult threats with difficult opportunities.
For example, a demon (classic) says he can grant them the power they seek, but they must do something heinous in return. This affects their character's alignments, and their abilities to return to some of their other quests but they are granted the powers they seek.
What are some cool other ways to involve a gestalt class naturally into the game world? Cool ways to make them more difficult beyond the first to a near impossible amount after x levels?