Mystery chests is an interesting idea, and what´s more, this time, I think rewards are good enough. Percentages are much better here (especially when talking about new set buildings - like library, which is quiet difficult to get in spire itself).
Anyway, I´m still not sure if we can consider this as improving rewards in spire - I´ve said that a lot of times and going to repeat myself once again - I do really miss ranking points here. Apart from my focus on them, this is a good way how can players watch others and how much are they doing in spire and that really helps to motivation (just like we do in tournaments).
Last but not least - I still await changes in battle background. It´s really weird fighting on a grass when reaching the sky. This is not something necessary, but I think it should be done before it hits live server, just to give a fully developed feature.