I'd prefer none of this, thank you very much.
I recall Larian stating they won't ever have backer-specific content in DOS2, to offer same game content for everyone.
Plus, it's not just the implementation and testing, it's the cost too. Don't get me wrong: it's grand and awesome 30, 000++ pple supported DOS2 KS but we're what? A pittance compared to the millions more who'll be buying OS2 later.
Now, if we're going to have an arena mode for everyone, which is quest-integrated? I'd say "yes" to the idea. It can be fun to have arranged fights versus a variety of opponents, for money, xp or some other quest reason.
I'd say Larian has learnt their lesson not to create a feature just 'cos "it sounded like such a great idea". Yes, at the risk of repeating myself, I'm referring to the Divinity2 video where they mentioned they were gonna turn the last 1/3rd of the game into an RTS. And oh yeah, they were gonna have tutorials for the newbies.
Given Larian and its lack of hand-holding, I'm really glad they never went through with it. Still, it explains why the Battle Tower was so empty.