An app store is a relatively easy thing to create. That is not a problem.
There are a number of problems in sustaining a viable Amiga software market.
To have a market, which promotes commercial quality software development, we need a larger user base to warrant development costs.
Aminet, OS4Depot, Morphzone and the AROS Archives, while extremely useful and necessary at this point, effectively detract from a commercial app store.
The current AmigaOS4 hardware is too expensive to provide significant growth in the user base.
MorphOS users are only in a slightly better position in terms of hardware accessibility, but will inevitably hit a roadblock with Mac hardware.
AROS users typically want their apps to be not only open source but free.
68k users are in decline, although Minimig and Natami have the potential to hold that steady.
The propensity towards free open source ports limits the viability of new Amiga dedicated software development.
Apps that support all Amiga&alike platforms may be limited to the lowest common denominator, but would have access the the super-set of these user bases.
Taking it further, Apps/games, based on a cross platform toolkit, which aside from Amiga compatibility, can provide executables for iPhone/iPad, Windows, MacOS and Linux have an even greater chance of commercial success by leveraging these larger user bases.
I see a requirement for cross platform software architectures, if a developer is to have any chance of making their development costs back.
I am currently working full time on such a platform agnostic application architecture, but out of necessity, I am focused on those other platforms first.