Just to add to the discussion
https://owncloud.com/owncloud-announces-new-marketplace
Th apps have to be free. The key is “subscription for maintenance/support”
@Rello where do you read the “apps have to be free”? Reads to me like everything is possible. Closed source apps would only be compatible with the enterprise edition.
it was my assumption from what I understood from Jos 
wrong wording then
Realize this is an old thread, but it was the one most relevant to licensing apps on NextCloud that I could find.
On the page describing publishing apps for next cloud (App Developer Guide — nextcloudappstore 2.0.0 documentation), when it mentions the license, it says the following.
licence
- required
- must contain agpl , mpl* and/or apache as the only valid values. These refer to the AGPLv3, MPL 2.0 and Apache License 2.0
This would imply that NextCloud applications can be released under a Mozilla or Apache license if desired.
So. in the case stated here by the OP, would it then be possible for this OP to create an interface app for NextCloud that uses, say, the Apache license. Then in that interface app they could allow function calls to NextCloud. The function calls could then be used by whatever application the client needs and then the client application would not need to be licensed under AGPL and, indeed, would not even need to have the source code released under any license. From what I understand, this is the approach that MongoDB took, where the database engine is licensed under AGPL3, but the drivers for the engine are licensed under Apache (https://www.mongodb.com/community/licensing).
Is this a right interpretation of what is allowed for NextCloud apps?