I really don't agree with the "you have to have these roles" viewpoint. If by fighter type you mean tanky, you can go without a tank through several methods (disengaging, kiting, CC and buff/debuffs). If you mean high damage melee combatant, there are more builds that can do this than can not. You don't need a dedicated stealth expert, and anyone with proficiency can pass required skill checks. A healer is nice to have, but you can make do without. Even with a cleric, I'm more likely to use their slots for buffs and utility, which several classes can do. Besides, a good number of classes can be built to be self-sufficient in the health department. A caster is definitely needed for specific problem solving scenarios, although I think you could get away with using scrolls or cheesing. They'd be more pertinent to the exploration aspect of the game, in which case I would be very surprised if Larian didn't provide numerous workarounds. On top of that, several classes can fit several of those roles anyway, so if you do gravitate to that layout you have several options on how to go about it.
That said, I also like variety and wouldn't be opposed to 6 characters. I have to agree that there far fewer hoops to jump through if you want to mix up your party composition when you have 6. 4 is definitely restrictive on your options for a party. It just isn't because you need to follow the fighter-mage-cleric-thief layout.