Is this just for the forums or for the deckbuilder or both?
I have no issues with the forums, but I have a few suggestions for the deckbuilder:
The ability to filter cards by color identity, not just color. And to be able to filter multicolored cards by their actual colors, not just by that their multicolored, i.e. searching for only BW cards. on the flip side, being able to filter cards excluding a certain color, i.e., filter out cards with white and/or blue in their color/identity.
The ability to filter with more than just one set. Say I want to pull up all the shocks, or the checks. the first requires at least two sets (guilds and allegiance), same with the other (dom and ixalan).
The ability to filter with keywords. i.e., contains "
exile target creature" (exact phrase), or contains the words "whenever enters battlefield."
The ability to filter supertypes and subtypes. i.e., able to search only creatures that are Legendary and have the Warrior subtype.
that's all I have on the top of my head for now.