Here's an easy one!
You know how it shows Cradle when you log in? Remember how it used to show the Clockworks and it was all pretty?
Well, wouldn't it be neat if the game randomly cycled between these and maybe a few others? Others maybe like the Jigsaw Valley background and the Night Slumbargo background n' stuff.
(If you want a reference for what I'm talking about, go play Warframe, and when asked for a password, just keep restarting over and over again. Assuming you have some progress in the game I think you'll see what I'm talking about.)
Would be interesting. Would be cool if it took the rank of your highest or last played knight into consideration - such as the skylark being outside of cradle when you first launch up the game, or maybe watching escape pods drop onto cradle.
Subtowns might be a nice view too. When events come around having apocrean, black kat, and tortodrone themed backgrounds would also be cool.