We'd have to be able to see via the dashboard that the AV is functioning and updating. Doing that would require the devices to communicate with the Kabuto dashboard on a more regular schedule, at least hourly.
The ability to re-run those AV related checks on the fly would also be ideal.
We'd also need to be able to trigger updates, scans, and change settings right from the dashboard.
Being able to add exceptions from the dashboard would be awesome too.
Emsisoft is pretty lightweight, but maybe having a system resources priority setting (low, medium, and high) would help. Especially for older devices. Of course this too would need to be able to be set in the dashboard.
Being able to install and uninstall the AV directly from the dashboard would also be a must. Kabuto should also be able to automatically forcibly uninstall other AV software prior to Managed AV installation.
Since Emsisoft is an AV with anti-malware and basic web filtering all in one, the need for an additional web filtering app would be pretty much non-existent, making life a bit easier.
As far as price, Emsisoft by itself if super affordable, so we wouldn't want to charge our clients too much for it, lest they just buy it on their own off the internet.
Maybe a buck or so a month per device would be a decent fee. Shops could easily resell it for about $5 per month ($60 per year) as a "premium" service since the AV is managed.
...You asked for pipe dreams, so here you go...