Main things to consider
(a) marketability of a qualification .. now and later. While we can trot out the trite expressions about the qualification's being useful in itself, the reality is that some qualifications are not particularly marketable when it comes to getting a paying job .. PhDs in needlework and similar come to mind.
(b) is the qualification liable to go stale unless you work in the discipline in parallel with flying ? If it's liable to go stale how difficult is it to get back up to speed and become marketable.
(c) does it suit working part time in parallel with flying ?
(d) underlying interest in the discipline.
Examples I can bring to mind ..
(a) a long-retired chap who, as an ANA F/O in Australia, qualified in medicine .. subsequently gave flying away and went into medicine full time .. qualified in electrical engineering and ran a successful avionics shop on the side. He had a 685 (if I recall correctly) .. did some design work on it at one stage and traded that work for a hour or two at Essendon .. he was in the RHS (to make sure I didn't kill myself) for the flight but wasn't used to tight circuits within the aerodrome boundary .. a very nimble aeroplane with only two on board.
(b) I qualified in aero engineering .. then, after getting into airlines .. was approached by DCA to do some engineering work on the side .. led to quite a successful and interesting parallel career.
(c) a contemporary in aero eng. Worked for DCA and then into airlines. Post the 1989 stoush, flew overseas. Eventually ended up as CEO (if my recollection is correct) in a significant third level airline operation.
(d) colleague who qualified in Commerce (as I recall) then did a serious MBA. Led to a useful parallel airline management career.
(e) two colleagues who qualified in law. One, I gather, has more or less moved out of flying but is still involved in aviation law. The other still runs a successful GA training operation as a parallel business.
There are a few others I can bring to mind but you will get the idea, I'm sure.
Problem with disciplines such as engineering and medicine is fitting in the shop front practical work for the undergrad subjects in parallel with flying .. can be done, but expect some sweat along the way.