I played for another half hour and came back with more thoughts. Would be awesome if you could slowly rotate your amalgamation around its center of mass by holding Q and E (perhaps turning more slowly the more cells you have). Dead cells should be grey rather than blue, and it would be terrifying but kind of awesome if you encountered enemy nuclei which could attach to dead cells just like the player as a kind of boss. Small pauses inbetween waves to arrange your cells would be nice as well.
I think this game is a hit, and if you're interested in developing it a bit more you could probably make some money.