I know I have a topic about this from quite a while ago...but considering not only is the topic really old, but the fact that I hadn't done anything with the project in so long, I figure now that I have a little more to show, that a new topic is in order.
My game project is a top-down space shooter, with 2d gameplay but 3d graphics. Attached is a video showing current progress. The ship model is temporary, and the energy and shield tank GUI on the left is as well(the "holder" parts anyway). The main Roid Buster weapon is working, and though it isn't in the video, the upgrades for it are also working. That lightning is the level border, meant to keep aliens(and you) in the area until you get them all destroyed. The left tank is the shield, and the right is the energy(used for shooting). The little lights above the shield tank will indicate extra shields(a shield will refill little by little, but once destroyed it doesn't come back that level, so extras are good so you don't die).
The game will have some decisions for the player to make as far as what to level up. This includes upgrades in capacity of shields and energy, refill rates, extra shields, and then each weapon has things like cooldown rates, etc... The design isn't set in stone, but my thoughts are along the line that you won't be able to replay levels just to grind it out. This means that you have to choose wisely what you prefer to upgrade because you will only get so much as far as upgrade materials to spend. I am considering making it so you could downgrade later in order to upgrade other things in case you made a mistake...not totally convinced either way yet.