If your GM wants to allow for additional creativity for experienced players (and potentially broken combos!), when heroes level up, they may choose ANY class. For example, when a level 4 Berserker levels up, he could pick Commander, and take the level 1 Commander features instead of the level 5 Berserker features. He would have 4 d12 Hit Dice and a single d10 Hit Die. A hero gains the equipment proficiencies of all their classes but should use the advantaged/disadvantaged saves of whichever class has the highest level.
"Broken" Can Be Fun! Planning, building, optimizing, and “breaking” a class build IS the fun for many people. The GM may need to make the game substantially more challenging if multiclassing is allowed. The GM also may veto any particularly overpowered, unfun, or implausible combo for the sake of the story and overall fun of the table. If this happens, well done! Your hero was too powerful for reality.