I personally love Zilker, but my love of it doesn't make it qualify as a world-class park...I know, I'm not that big of a deal.
However, I think that one of the charms of Austin is the natural settings of its parks. A lot of parks in massive cities around the world that I've been to seem a bit sterile and manufactured to me...though a deck park over 35 would definitely allow for some of that and would be an appropriate setting for it also.
I am hopeful of how the Waller Creek Trail park(s) will turn out - those have great potential.
Back to museums. I think a Live Music Museum would do incredibly well, so long as it's not a cheesy collection of stuff a la Hard Rock Cafe. They would get so much traffic during ACL and SXSW alone, not to mention normal traffic throughout the rest of the year.
I echo the benefit of expanding Blanton and the addition of a proper aquarium and zoo. Would a zoo fit well into Zilker or in that area? Or should it be left out in the sticks where the zoo currently is? Although that one is more of a rescue shelter, it's still great for what it is. We also have the LBJ Library and the Bob Bullock...those are decent as well, but having other options would be great. A planetarium across from Bullock is still my preference, even without the tower.