The idea is quite complex. Ancient city portals will take you into a new and temporary dimension each time they are activated. The dimension is an enclosed arena-like dungeon that is around 250x250 in size. When you light the portal, you will be transported to 1 of N well-designed arenas, each arena with it's own designated unique boss. These bosses would be like no other each with very unique dangers. These arenas would have cool unique blocks/items only obtainable in that dimension. The concept is so, that until the boss is defeated, you can not return. But, as a unique twist, once the portal has been activated, it will close within a short time-frame. Meaning you can not re-enter pick up your loot upon death once the portal has closed. 

This would be a very difficult mechanic, only for the best of players with the perfect preparation of gear. It would add a whole-new ceiling into the game. Ideally, the bosses would be designed in such a way where preparation is key and without bringing the right resources winning is near impossible. Minecraft for some time has needed something more challenging and I believe this would bring it without ruining core game mechanics.