It's not really my idea, but whatev.
So, let's say you have a persistent world populated with player characters. Type A, call them Zerg, respawn easily. They have a little customization and no death penalty. Type B, call them Protoss, have a big death penalty. They are also very powerful and require time and effort to level up.
Now they fight!
There's a hundred different combinations. Zerg could be free to play and Protoss could be pay-to-play. Zerg could be console gamers and Protoss could be PC. Zerg could be level 1-59 and Protoss could be level cap. Zerg could be the PvP class and Protoss could be the PvE class with the option for PVP. Zerg could be twitch and Protoss could be stats.
I'm pretty sure it would be fun to fight something that could destroy you easily, but you can respawn and come back for more instantly. I can't really think of an example except for deathmatch. But deathmatch is different because you get some frags between deaths, whereas Zerg vs. Protoss wouldn't, really. Maybe Zerg could disable part of a Protoss like in certain mech combat systems, so you'd have intermediate goals.