It would be nice to see a standardised set of proprietary licenses that you can easily dual license a copyleft project with.
Given the way that SPDX is set up, I imagine the ideal way to do it would be to create some standard proprietary licenses and then add exceptions for "small" users that don't want to use the copyleft license and would eventually like to get big.
i.e.
- Commercial-Royalties-1pct
- Commercial-Royalties-5pct
- Commercial-Royalties-10pct
- Commercial-Royalties-Negotiable
- Commercial-Seats-Negotiable
with the following optional exceptions
- Under-100k-USD-Revenue
- Under-1M-USD-Revenue
- Under-5M-USD-Revenue
- Under-10M-USD-Revenue
There's a certain amount of inflexibility that comes from trying to standardise license terms in a way that's compatible with SPDX but maybe an extension to SPDX in the future might improve that and allow you to specify certain terms on licenses more than just basic exceptions.