Path Of Time
Path Of Time is an interesting vertical shooter with time traveling and a minimalistic graphical style.
Collect enough sand to fill up your hourglass and then activate it to support yourself with another instance of yourself for 10 seconds. Some kind of an 1 player co-op mode. There are also boss battles at the end of each stage that need some time travel support tactics. After you have beaten a boss you can buy some upgrades in the shop.