How about leaving the PADI fold and moving on to other styles of diving, for example technical diving: where you can learn to do much longer dives with decompression (an hour at 30 metres?); where you can use alternative gases - Trimix - and dive deeper; where you can investigate the amazing underworld in caves/caverns/mines; diving with CCR; diving with Sidemount...
PADI are great at getting you started, but they're not that well known in the technical diving world where the likes of TDI, IANTD, GUE and others rule the roost.
Becoming a Divemaster is fine, but is utterly meaningless in technical diving where excellence in buoyancy, finning, trim, decompression procedures, dive planning are all essential and are required at a far higher standard than required for the recreational Divemaster qualification.
Backfinning? Helicopter turns? Frog kicking? Valve drills? Hovering motionless at 6 metres? Ignoring the NDLs? Understanding decompression theory? Accelerated deco? Holding your stops? Not to mention the endless scope for buying tech kit! None of these are covered in the Divemaster course.
To answer the solo diving question: what's wrong with diving solo?
Imagine you're on a boat and the 'captain' foists some insta-buddy on you. You're then committed to diving with literally anybody; and it's very common to have a novice buddied to you -- they get an experienced diver; you get a liability. Fine if you like that kind of thing, but haven't you paid for a dive too? Provided you've the right experience, skills and redundant kit, why not dive on your own?
There's a couple of solo diving courses, such as the SDI Solo Diver course -- that was the best fun I've ever had on any diving course: being blindfolded, asked to follow a course around some submerged ISO containers by touch; tied up (there was some rope the instructor found and dropped on me), air gunned (i.e. simulating a freeflow - so valve shutdowns), having a fin removed and then deploying an SMB then doing a controlled ascent with stops whilst still blindfolded.
Interestingly the SDI Solo Diver has a pre-requisite of 100 dives, which is twice that of the Divemaster. The reason is that you've got to be on top of your skills - you're not learning new skills, you're learning how to use your existing skills.
Is there any reason why a non-professional diver should not have a divemaster cert?
Because it's fundamentally meaningless and the skills aren't particularly useful outside of training or within the PADI system - c.f. the tech skills above