It would be very useful to be able to combine ipv4 & ipv6 rules into one ruleset. Nftables has support for this using the inet-family.

Support for this would considerably simplify firewall rules in a dual-stack environment.
This/similar functionality has also been requested in the forums a few times: 1 2

I don't think any automated configuration migrations would be necessary. Instead (like nftables) all variants should be supported (ipv4/ipv6/inet).
However, perhaps it may be possible to simplify manual migration from a ipv4/ipv6 based rules set to a common inet based ruleset. Maybe of some sort of rename or copy of ipv4/ipv6 based rulesets to inet based rules may be possible. Not sure how one would handle rule number collisions though.