The snowdon aviary required an estimated 1 million spent on it to rennovate it even as far as 20 years ago. To my knowledge, the money was never spent and it's inhabitants have slowly declined over time, with the exception of the ibis which breed and the birds of prey (first bateleur eagles, then kites). The aviary is so iconic and has great potential, but would be hard to re-market as anything perceviably different to it's current function, and therefore I imagine rather low on the list of priorities for capital expenditure. Plus, ZSL have created some wonderful walk-through aviaries in the last couple of years that are far less harsh and more visitor-friendly than the snowdon.
The slope on the North bank is steep only in a few places. Most of it is very wide, and flat along the path, with many young trees and scrub growing along the length. The path is quite wide and if replaced by a raised platform through an exhibit, would be a very good size. The space at the bottom of the snowdon running along to the education centre is also not inconsiderable.