With the options of env_sprite (rotations, mask, transparency, etc)
Tile animations are just way too time consuming to make especially with 320x320 sized files.
Breakables that only one team can cause damage to: (example: barricades and zombies, want the survivors to shoot over the cade but zombies to break it)
Sliding tiles: Tiles that move your player one direction like a conveyer belt with changeable directions and speeds.
Better loops (instant loops): Like the audacity program, we need sounds that loop better (instantly), music is very hard to get sounding good in cs2d.
More editable shadows:
More map specific stuff like changing the shadow direction, length+darkness, toggle (on/off), and overall darkness of the map (without using a sprite with a transparency to make your map darker).
Shadows and lights that can cut through designated env_sprites.