Answering the conceptual bit as a separate reply to not mix it up with the technical details.
Conceptually, do you envisage the development of Multiple MarketPlaces (by different providers)?
Not in the near future - you may remember our discussion a while ago on managing applications/dataplugs and a sort of an app store and due to lack of commercial interest HATDeX decided to implement it as a necessary service. All certified HATs have to be registered with MarketSquare as do all certified dataplugs. "Certification" for now is a very light touch just making sure it looks reasonable and trustworthy, but the quality control and policing is very important in the personal data space, so we don't see it going away, nor we see multiple MarketPlaces with the official HAT ecosystem, although some of that may be delegated to partner organisations as we all build up capacity, but not completely without any oversight.
If yes, dataplugs should be runnable in all the market places?
So the short answer is no
If so, is there a specification that all market places must implement to be HAT/Marketplace compliant?
No, but we can get a discussion going on the delegation side of things.
Hope this clarifies.