What can we improve?
Flexibility and ease of use

syntax cleanup
Make sensible defaults implicit
'flags S/SA' and 'keep state' now default
first-match nat vs. last-match filtering
inconsistent syntax for minor options
terminology cleanup
prune or remove route-to/reply-to etc
largely obsoleted by routing code changes like multipath and multiple routing tables
removing these will also improve code readability

This is difficult to do without breaking existing rulesets.