Well you've laid out your problem. Whatever you decide to make, it's going to take a lot of work to have it compete with what's already out there.

You're going to run into the same issue of 'been done before' on the Asset Store, only worse. Unless your platform systems are better than More Mountains Corgi Engine, or your adventure game system is better than Adventure Game Creator, it's not going to sell.

You actually have more chance of success with a derivative game on Steam (people are bored and looking for the things they like that they want more of ) than you do with a derivative asset on the store (people want something that not only works, but works well).