Well at least roles don't match their titles.

Assassins: They'd actually one-shot you with a sniper that covers the entire map with its range, and be able to poison your health pots with a fast-acting neurotoxin.

Tanks: They would be nigh indestructible except by each other, and also one-shot everyone else all at once.

Supports: They would have to wait in the fountain or queue for an extended period while they study medicine. That, or would be stuck sitting at the base relaying information to their allies.

Fighters: They would need a referee around to have a match, and would be disqualified and given a two week ban if they kill the enemy during it.

Mages: They would only be able to perform illusions, nothing that benefits anyone in combat.

Marksmen: They'd also opt for snipers, or simply be taken out by snipers if they're the idiots who brought pistols and shotguns to a sniper fight.