Well I've beaten all but the last two levels (which I played and lost multiple times) and I have a couple suggestions/ideas.
1) Difficulties. Easy mode would have less aliens, hard mode would give them more health... it would make beating the game easier, and give it a replay ability.
2) Achievements. Like suggestion #1, it would give replay ability. Achievements are fun.
3) Upgrade tree. Like xodarap777 said, Second Earth feels like a puzzle game. Adding an upgrade tree, where each upgrade opens a new branch of upgrades and closes another, would add greater customization and let strategies evolve as the game progresses.
4) Sandbox and survival modes. I personally love sandbox modes, because it lets me see what the game is really capable of. And survival modes, where you're faced with an infinite amount of increasingly harder waves is also pretty cool.
5) Flying units. Regular turrets are pretty much useless in late-game rounds. Something that can only be targeted by them would balance it out.
6) Variants of the basic turret. Again, basic turrets get obsolete as the level progresses. They're useless against swarms, and don't have the dps to kill stronger aliens. And they're too inaccurate to kill kamikazes. A machine gun turret would make for better crowd control while a sniper could pick off kamikazes and kill tanks.
7) More/better traps. Stun pads are useless. Because they only stun one enemy, they're no good against swarms. And tanks (the aliens you really want to stun) only ever appear in swarms. Mines would be useful.
8) Mine boosters. Just like you can boost community centers and farms, you should be able to boost mines.
Yeah this is long, but Second Earth is a really great game. It's got a ton of potential and is pretty much the only game of its genre, so i really want to see it on Steam in a few months.