Going back to the plague... maybe an idea from Star Trek?
Maybe the plague did ravage the spellcasting community, but the drug used to treat it wiped out the disease centuries ago. The drug is also highly addictive and the withdrawl symptoms look like severe viral infection, leading people to think that the drug still cures the extinct disease. The leadership of the country keeps up this appearance to control their own mages and to make free mages turn themselves into addicts willingly. Sounds like a really cool political corruption hook if I ever heard one.
Present it like a disease and require the fort saves as above, but four (or however many) successful saves means that you survived withdrawl and take no further damage unless you're administered the drug again.
This is fantastic.
I'll go with it!
I'll also add in the political power plots, which are good, and the sudden outbursts of wizard mayhem when one of the wizards gets his sending of "the antidote" delayed.
Also, I'll add in a few low level plots that's about delivering the anti-dote to a half-mad wizard constantly putting the adventures onto odd challenges.
Also, the cabal of Necropolitans and Constructs sound great, my world already features an ancient civilization gone extinct, who boasted warforged, necropolitans, liches and Elans among their ranks, this cult could be some new-comers who have found the ancient scriptures and replicated it, making them undead and therefore immune.
However, since damage to the mind doesn't really heal when you become undead, all these guys would be at least half-mad. And therefore they'd often do things that seem rather evil, although their goals might be quite noble.
Oh, and Undead are outlawed in my campaign setting, so they'd have to be rather undercover anyway.
This is gonna be great.
Two stage onset?
A rapid phase of insanity, followed by a slow rotting alive.
Yeah, this is sounding awesome. I totally want to run or play in a campaign with this, now!
Let me know if you start a PBP game with it, either I'll join as a player, or I'd very much like to know how it goes.