Agreed, there's a lot of flexibility in party composition in BG3, as a result of the flexibility in 5e.
You don't need a rogue, as long as you have someone else who can pick locks and disarm traps, which could be anyone with decent dexterity, though ideally also with sleight of hand. Any class with the urchin or charlatan background, or a ranger with Urban Tracker, or a bard, can get sleight of hand. They aren't going to be as good as a rogue with expertise, but should be good enough. In Early Access, if you don't want to take Astarion (the only rogue), Shadowheart as a trickery cleric with urchin background has good enough sleight of hand if your own character doesn't.
Of course, rogues can also be stealthy scouts, but this is optional anyway, and other classes have ways of accomplishing this, such as a stealthy ranger, mage with invisibility, or sending in familiars (the cat is sneaky) or animal companions, or for a druid turning into an animal themselves.
And you don't need a specialist healer. There are plenty of potions, which can be thrown to heal others if a character can't drink themselves (though I think this is dodgy so I tend not to do it). But plenty of other characters other than a cleric or druid can get the odd healing spell that can cover your party if you don't want to throw potions, eg rangers, bards and paladins.
As fylimar said, you don't really need a tank either. You can get away with being mobile on the battlefield and using crowd control effectively instead. Or using summon spells like Flaming Sphere or an animal companion to help keep your weedier characters being overrun.
And while you are probably going to want to have a magic user, you don't need a specialist mage. As well as Gale (the wizard), there's Wyll (warlock) and Astarion can become an Arcane Trickster at level 3, and Lae'zel can become an Eldritch Knight, plus there are lots of options for your main character to be able to do AoE damage, crowd control, etc.
I actually really like the 5e approach and the 4-person party limit, as it forces me to be creative about how to cover the various functions my party needs, or find different ways of doing things when I can't cover a function well. Not everyone is a fan though, as you can find
a big mega thread on this forum saying!
That said, I take Shadowheart more often than not. Shadowheart can heal and lockpick and has spells to help stealthiness, so can fulfill both healer and rogue roles, along with some decent spell damage and buffing, plus if given the right armour she's not too squishy on the front line, giving me lots of flexibility with the remainder of my party. And I take Lae'zel least, just because I'm not particularly attracted to fighter as a class, and have found that the fights are all perfectly winnable (if sometimes trickier) without a tanky front-liner.
