Eh, might as well repost my idea since we’re discussing this again apparently. :eek:
I’d rather something like this, than having a short timer dictate when new enemies spawn - I’m not a fan of the idea of being thrust into potentially unwinnable situations if you take too long. We already have a punishment for going too slowly, after all - less loot.
Then again, fighting only two bosses at once as per my idea might be too easy, especially since you’d have the whole arena at your disposal. Hard to say without testing really. Would depend on what those two bosses were as well.
If you’re on Ultimate, just do what I did - drop down to Normal, get through the Log shards on that difficulty to unlock the 65+ waystone blueprint, then go back to Ultimate again and jump straight to 65. 
Or just wait until the next (presumably) patch where the Log arena is having its size increased.