An endless mode where every 5 rounds we fight a random boss from D1 strikes all the way to current strikes, then every 10 rounds is a double boss encounter. Any mechanics tied to those bosses would still be there, fighting the wizard boss from the Dreaming City strike and one of the shield brothers at once for example their would still be the ball mechanic for the wizard boss. Bosses could even summon tougher variants of current enemies that have a new attack or something. Their could be modifiers that also change every 5 rounds. Field traps that also change every 5 rounds with the field traps being more lethal during boss fights. Like a laser grid that moves around as your fighting the double bosses or something. The farther you go the better the chance at higher stat gear. The map could be a colosseum looking arena as well.