I have the strangest feeling that when the mahjarrat and vampire quests are complete there will be a huge lack lore to keep runescape interesting. The only big piece that I see that could come from the mahjarrat and vampire story lines would be something to do with the elder gods. We only know of one elder god thus far which is Jas. I could almost forsee us being involved in quests that show our current gods as being usurpers to the elder gods.
To be fair:
Jagex have confirmed ROTM isn't the "end" for the mahjratt.
It just ties up the on-going ritual story arc, and according to the 15% membership news post ties up Lucien's plot.
We still have several big mahjratt based plots to bubble along nicely AND jagex did say somewhere ROTM will actually start off 1 or 2 new mahjratt stories (eg sliske).
I mean there heaps of mahjratt stuff left to do.
Hazeel's next step, Azzandra restoring Zaros, Jhallan post-ritual, Zemergoul's next move in his quest for varrock and us nicking Arrav's heart, Enkhara and Anthankos on-going feud, General Khazard being stuck in the shadow plain and hating us, Bilrach's dealings under daemonheim.
They didn't say RotM would give way to new Mahjarrat storylines. If I remember correctly, Paul's devblog said that the quest will give rise to several interesting overarching storylines only touched on previously. One of these is undoubtedly going to be the Dragonkin.
Also, I don't really see much point in having individual Mahjarrat related quests post RotM and I expect to see most of the loose ends you mentioned tied up in the quest itself. Afterall, what's the point in having a quest with Zemouregal just attacking Varrock again only with added power.
The only Mahjarrat I hope to see with his own questline post RotM is Sliske, mainly because he has close ties to the Barrows brothers and Nex which would be interesting to see more of.
Also, if ANY of the the signature heroes are in this quest, I'll go find the dev that ruined arguably one of the best and only good storylines left in the game and put his head through the computer screen. (lolemptythreat)