Traps are fun, but notoriously hard to balance. Do enemies always walk into them like idiots, or can they somehow spot them if they're poorly placed or set right in front of them? Could be fun (for the rogue at least) in competitive questing to set up traps to thwart your allies. Trip wires and spring traps (something like a preset teleport spell, though probably chances for bugs) would be more roguish than elemental traps, poison and oil traps also more likely than other elements. Reverse-pickpocketing bombs that go off when the enemy takes damage also sounds devilish. And I think lockpicking should be expanded to device expertise and influence the efficacy of traps. So traps wouldn't necessarily be rogue-specific (would also make sense on rangers), but would fit well on one. Not sure if Larian will go this route. Can't even remember if they had traps as a skill-tree we could vote on.
And fighters are almost always a bit bland. They're front-line fighters that take hits and tend to deal with multiple enemies better than rogues. Stealth and backstabbing is fairly out of character for fighters, though maybe all characters should deal bonus damage to enemy backs? That's sort of the case with flanking reducing armor, but it's not a very large effect. Fighters also tend to be more team players than rogues, who often are just baiting the rest of their team. As such, a fighter and a rogue tend to work better than two rogues or two fighters.
