Just an idea, but i reckon it would be cool if you started at level 1 and gradually get levels up.
Depending on your level you can get different advantages and upgrades, such as-
1-10 Able to use s3 aliens
11-20 Higher swipe damage and more stamina
21-30 Scope on rifle or las gun- able to shoot longer range and keep accuracy.
These are just some ideas, i thought i would share it.
Please say what you think about it.