A good addition to the Grand Arena would be the ability to exclude units during the preview phase. This would allow a player to effectively bench certain units so that their GP is excluded from matchmaking, but of course they are not available to place on defense or use in offense during that Grand Arena match. I suspect this would have to be limited to a % of your total GP to prevent gaming the system (maybe 20% of your overall GP).
As a 3 year player, I have quite a few units which have passed into obsolescence. I geared up jawas, IGD, and ugnaught for Tank before it was released (doh). I have Luminara and Sideous at g11 from a time when they were somewhat useful. And I have some pilots geared which would be pretty useless in a character-only fight. I'd love to be able to bench them instead of them counting as dead weight. Even players that aren't in my boat would be able to strategize a bit more by narrowing down their rosters.