Make a pet class for easier/less repetitive soloing.
Pet classes are traditionally better at soloing, but maybe a little less effective at filling a party niche. For example, a bear and shooter guy can take more damage because combined they have 140% of a normal character's health, but the bear is only a so-so tank and the guy is only a so-so shooter. The bear tanks well enough for soloing though, but will get creamed vs. a squad of foes designed for 4 man.
Anyway, that's the main gist. Soloing is pretty repetitive, so make a pet class or two that give less chance of dying and more options when soloing, but will always be second best at filling one or two other roles.
I'm sure with your world's unique flavor you could make a good one that doesn't follow the tired tropes of hunter, necro, and druid.
This would also take some of the sting off not being able to run my own party.