For the classes, I'm drawing inspiration from numerous sources, including Destiny 2 and MMOs like World of Warcraft. I'm going from a perspective of the fantasy genre, so someone else might have take the sci-fi angle, as it's not my forte. I picture the Defender to be similar to a Paladin: a sword-and-board wall of steel that can taunt the boss and either increase their defense or lower the boss's damage. Perhaps they have an ability that gives the party an overshield that either a) lasts a fixed amount of time, or b) absorbs a set amount of damage before breaking. They could also have what I've heard called some kind of Thorns effect, where the enemy takes damage when they hit a party member directly. For the Attacker, they might be a fighter that goes in swinging, either a two-handed weapon or dual-wielding. You can also go the "WoW rogue" route and have them be a stealth-oriented stabby-boy. Offensive Support can be a number of things, but what I picture is something akin to the Bard from Final Fantasy XIV: someone that can increase the party's damage while getting a few choice hits of their own. Perhaps they maintain certain buffs like increasing attack speed or reducing cooldowns. Defensive Support could definitely be the healer. While the defender's overshield would help reduce the damage the party takes, this character could heal them back up with various spells. Hope this helps.