If there is demand from paying customers, we’ll build it. If somebody else wants to do it in their free time - well they can go ahead, companies love it if people work for them for free 
If you want to pay a mid-4-figure amount of dollars, sure sure, you can run whatever you want But as Kesselb said you need a second server that runs the Kaspersky Scan Engine. We don’t intend to ship that, as you can imagine
Thanks for the list 
To integrate a virus scanner into Nextcloud an application programming interface is required. That’s usually a feature only available by paid (enterprise) solutions.
If one knows a free virus scanner with an Api please report it: GitHub - nextcloud/files_antivirus: 👾 Antivirus app for Nextcloud Files.
There is YARA for open source malware detection. I see it is supported by the open source Armadito antivirus project, who also use ClamAV and allow external integrations for antivirus detection.