Nobody said it would be easy. On the other hand, there is Steam, which is an app store on Ubuntu.
The Ubuntu One store had technical issues and Snap solves at least a few of those if not all. For example, "works only on Ubuntu!" or "what about my dependencies?". It would be hilarious if Redhat users would buy their proprietary software from Canonical.
Still, the main challenge of an app store is not the technical side, but the business stuff: Get a critical mass of apps. It is also the reason why only Canonical or Red Hat could pull this off. It would be quite the hustle. Even Steve Jobs had some fights for the Apple App Store.
Some ideas to put there: Sublime Text, Gitlab EE, CLion, PyCharm, Zend Studio, Gurobi, Spotify, Netflix, TeamViewer, Matlab, Mathematica, SPSS, Stata, Maya, VMWare, Crossover Wine, Guitar Pro, Bricscad, Houdini, etc.
Another problem is the barrier of entry. How to get the customers to open an account? Once the account is open, buying stuff is one click and the 1$ apps will be bought on impulse. Cooperate with Humble Bundle and others for exclusive sales. The usual PR drill.
There are opportunities like App Store for Business. Where the company buys the IDE for all its employees via the app store and handles all the licencing there.