Even in a small network one will handle devices or rooms with a reservation system. Has anyone an idea, if there is such an app planned or in development? In an online-research I’ve found nothing like that.
No app planned nor in development 
Generic (and very blunt) answer (no offense): people don’t want to code your enterprise software for free, either do it yourself or pay someone to do it 
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OK, I see your point. But I did not say that I’m looking only for free apps. I would pay for such an app. Changes anything?
I don’t think there’s an official platform (yet) for developing custom apps. @jospoortvliet any ideas?
Range types in PostgreSQL can do all the reservation logic for you. Thus, it’s a minimum of programming effort. Just add a few lines of PHP and there you are:
Enterprise support is one way to get required features into Nextcloud. If it is just for a feature, you could ask some developer in the freelance sections https://help.nextcloud.com/c/nextcloud-freelancing