(Most of this is from the frame of trying to reduce expected opposition.)
The potential success of most example policies or policy areas I can think of are going to be highly dependent on region and political milieu; for instance, animal welfare measures have a good chance in Berkeley, California but not Ames, Iowa.
Potential, non-region-specific reforms would be ones that aren’t (yet) strongly red or blue coded, such as approval voting, which I’m moderately bullish on.
One way to reduce expected opposition is to focus on locations under single-party rule going into 2021 (or whichever year you’re reading this) and pick policies that match that party’s brand but are currently unsexy or non-salient enough that no elected official will care enough to champion them unless a constituent suggests it. E.g., even though Democrats won all federal elections in New Hampshire, Republicans will have control of the state executive and legislative branches, so this would be a good biennium for occupational licensing reform, which isn’t particularly exciting or topical.
Besides minimizing expected opposition, another way of minimizing expected effort is to find someone who’s already planning on pushing a package of reforms, and ask them to tack on just one more related thing that has good evidence behind it.
In areas with split-party leadership, I recommend going with one of two strategies, each of which has its own corresponding policy space.
Boring. Pick something dry and technical that has outsized effects, probably something in the regulatory sector. The goal here is to fly under the radar and not attract attention. Plenty of bipartisan bills pass this way.
If you can’t be boring, sidestep expected gridlock by picking an issue that cuts across party lines. Something that splits Republicans so that some of them will vote with the majority of Democrats or vice versa. This is going to depend on what preexisting fracture lines lie within your local or state political parties.