...what extension cabs might sound better with the combo than the Mesa extension cab?
If you really dig the Scout combo now, seems like a good choice would be more of what you already like (an appropriate/identical match to that cab). Nothing will
look nicer than that combo on top of a matching extension. Sound, well...
Depending on what is your version of "better", seems like you have a lot of (possibly lighter) options.
One guy has posted often about liking his WA Scout 12 with the AE112.
If you were going for that vibe and wanted new, people usually point to the ML112 for the same tuning (big low+high mid) ballpark as the AE. And I have read much more about the WA+ML being a great match.
My version of better is a bit lighter, and tighter/punchier while still retaining what we all seem to refer to as the "old school" vibe. (Not sure I truly know old school, my oldest amp was mid-80s Peavey).
The Genz NX2 12 cabs, to me, are better. Compared to this and other cabs I had around the same (Berg. HS210, Schroe. 15+L, EA Wizzy), the
Scout cab just seemed a bit sluggish, slow-responding. Staccato fingerstlye, fast runs, fast picking, etc. was tougher to execute with the Scout cab.
That's what I think about when everyone else here uses the term "punchy". The Genz neo line voicing is a bit more "old school" I think versus clear/modern designs (like Berg. HT?), and definitely "punchy".
The NEOX models are pretty easy to find used. But, if appearance is a concern, you'll be pairing carpet with tolex (forget about finding the NX2 in a hurry).
Assuming your combo cab is the standard 4ohm, despite what anyone says about it being okay I still would not pair an 8ohm 12 with a 4ohm 12.
And I have better results using a single med/large cab over pairing two smaller to make a med/large. I would say buy a 212 you like and get used to moving that head in-and-out of the combo cab.
There is a used ML212 here in the classifieds. According to read reports, with the Walkabout head this would be a monster. Wish I had a healthy spine and hefty wallet hammer.