A party is easier to manage when each character has a clear job and the group covers several different kinds of problems.
Define who can handle frontline pressure
Someone should be comfortable occupying dangerous space or protecting more fragile characters.
Keep dependable ranged options
Ranged attacks help when positioning or terrain makes melee access difficult.
Cover recovery and utility
Healing, control and exploration skills reduce the need to solve every problem with raw damage.
Avoid excessive role overlap early
Shared strengths are fine, but too much overlap can leave the party unable to handle certain situations.