You should check out GURPS. Its a simpler system with universal campaigns (modern, fantasy, mech, dimension hopping, steampunk). The system is super easy. You start with 100 points to make your character. You can spend them on stats, skills, spells, and perks. You can even gain more points by taking quirks.
You roll 3d6 for everything. Your goal is to get under your skill number. Fireball of 13 needs to roll under 13. If its raining or something, your GM can choose to put a -4 on that. So now you need to roll under 9. Just simple addition and subtraction, but it works really well.


My first thought. Seems appropriate.
You can get away with it while having some downtime in a village. The bard is making coin in the tavern and the barbarian is drinking in the same place, the priest visits the local chapel, the warlock looks to spend some coin on magic baubles, etc. This also increases the creativity in which you can give your players their next quest.
But once you're out adventuring on that quest, you're a goddamn party. If you don't want to be a party, then go home and play a single player game.
Edit: I have had good DMs separate the party themselves though, but we always spend it trying to find each other again.