That does sound like it might be useful to create an 'expectations' document of some sort that explains what you just said, that you aren't designing the campaign around the idea that everyone will eventually have every skill... In many cases the point is that no one *has* the skill, so you want the players to figure out alternatives, e.g. faced with a simple locked door, the players might decide to use the Mechanics skill to take it off its hinges, the Explosives skill to blow a hole in the door or in the wall next to it, the Acrobatics skill to leap to the roof and find a skylight they can get in through.

As for a shared concept, you mentioned a merchant with a pack brahmin in the other comment, so that might be a good one: they're a trade caravan. Maybe one or several NPCs who might give them initial direction, but eventually bite it, leaving the players responsible for figuring out What Do Next. "Kid, I'm trusting you with this bunch, you got what it takes..." *croaks*