Pretty much anything could work, although I would say it works better under a type theme: Say, two factors.
So you might have an "eco-terrorism" campaign with only rangers and druids fighting against the rise of an industrial power.
You could do an Ars Magica troupe style campaign with wizards and fighters or rogues as companions.
Or a tribal campaign with barbarians and druids (or warlocks or sorcerers re-skinned as shamans).
Or an urban all rogue campaign, with multiclassing into wizard or sorcerer allowed.
Or maybe all dwarves and elves (or simply non-humans) aligning to fight the incursion of humans into their lands, so humans become the new orcs.
Etc, etc.
When you limit it to one factor, it narrows the themes substantially. I think the best way to start with one factor is just that: start that way, but allow for divergence, mainly through multi-classing. So you could have the PCs all start as street ruffians (1st level rogues) and then find opportunities to diversify their skills, within a limited number of options. Maybe one has latent magical abilities and takes on a level or three of sorcerer. Etc.
Or you could have the party start with only non-magical classes, because magic is outlawed by something akin to the Magisterium of Philip Pullman, and then force them to take a level in sorcerer or druid or wizard as their latent magical abilities manifest and they have to navigate hiding it from the authorities.