I have two suggestions I think would be a solid addition to the client.

First, and most important. There is no way currently in the client to search for keywords based on mechanics that use only a symbol. For example, the energy mechanic cannot be search and filtered through what is currently available in the client. This makes deck building tedious and difficult. Myself as well as others would prefer to either add the symbol in as a filter option or associate the symbol to the keyword it is referring to.

Second is more for cosmetic reasons. I have a pretty substantial collection of sleeves on arena. I would love to see a feature added to the client that would allow the users to organize their sleeve collections rather than it auto sorting by first acquired. Even if its just the order in which they appear. Folders of sorts would be even better.