Roster Help!

Right now I am in the 2* to 3* transition. Here is my current roster http://mpq.gamependium.com/rosters/Grem/

Got a lot of covers in the rewards I don't want to expire: 3* cyclops red 3* black panther blue & yellow 3* sentry green 3* vision red 3* doc oc black

My alliance isn't great and doesn't place high enough for me to get consistent coin to buy roster spots. Is there anyone that I should delete? Do I bite the bullet and buy roster spots? Any advice is appreciated!