I like the idea of secret special levels. But not as a prize for achieving one of "race" ranks. If this level would give something useful to the player, then this would mean that players with higher "race" rank would have an easier game than players with lower rank. Game difficulty shouldn't be modified by amount of killed monsters during your last runs, imho. If this level won't have anything useful(even exp), then after visiting it 1 or 2 times, all players will simply ignore it. Is there are any reason to create useless uninteresting secret level as a prize for "race" rank? Only way to make useless secret level interesting is to make it not ordinary(fun level full of teleporters, where you should find your way to the exit, or level-shooting range, where you get all weapons and mods, and a lot of ammo, but after exiting, you get back your inventory that was yours before this level, or idea, suggested by RickVoid, or many others).
If not "race" rank prize, than this level could be easter egg and useful at the same time. The only requirement is to hide it quite well. I remember level for original DooM(Evilution, or Plutonia, i'm not sure), where you are finding your way out of labyrinth full of AVs. There are possibility to create such level in DooMRL, and where after right combination of pressed switches(all over the labyrinth) you gain access to stairs to that special secret level. So you could solve puzzle by yourself, but also you could reach certain "race" rank, after what you get tip, how to solve it.