So I had this idea when I first saw the Survival button pop up in the Tavern. I thought: "OH MY GOD THATS GONNA BE AWESOME"
To my disappointment, the waves weren't endless. And there was a crystal. I mean, survival is about surviving, not protecting. But since Trendy insists, I thought maybe they could have survival, pure strategy AND holdout.
Holdout: No crystal, no time limit, endless waves, everyone has one life and get's one more for every 5th wave survived - AHEM - held out and maybe slow difficulty progress (i.e. wave 10 on easy is still easy, but on wave 20, the difficulty increases).
For the progressing difficulty one, the difficulty you pick will alter how tough enemies gets as you progress. If you pick nightmare, it should stay nightmare with only few (15%-20%) increases in enemy stats every X wave.
You: No crystal?
Me: Yeah, so people will have to work together to defend eachothers instead of an object.
You: And the time limit?
Me: The reason Über isn't good enough is because the game can end before anyone dies (provided they survive).
You: And the endless waves?
Me: Self explanatory. And if not, let my just say that not everyone likes to hit the limit of waves the game can reach.
You: And the lives?
Me: To reward those who are persistant enough, dying on wave 99 with only few enemies left would feel horrible afterwards, but if they gain lives as they progress, they get a second chance to finish the wave. Lives carry over, but are lost upon respawn.
You: Oh. But what about the difficulty progressing?
Me: To allow players to find somewhat better loot should they reach wave 75, and to keep the chellange up, increasing the difficulty like on Survival (but endlessly in Holdout), only skill is what it takes survive a wave.
TL;DR (Too Long; Didn't Read)