I bought a Futuron monorail (set 6990) a few years ago and it is a very neat LEGO set. That being said, I am not sure this theme would sell well these days. Due to the track and electronics (even if using existing PF), the set would need to be expensive to ensure traditional profit margins are maintained by TLG. I would struggle to pay $250 + for a NEW monorail set as an AFOL. Part of the appeal of the monorail sets is that they were virtually unobtainable as a child – too expensive, hard to find, short lifecycle, etc.
As an AFOL, now I can afford to spend the same $250 on a used old monorail and fulfill that desire I never met as a child. Sure it costs a bit more than I care to spend, but it is part of the appeal and challenge to me. The same goes for older 12v trains – many were never available in the USA and now I enjoying collecting the 7745, 7740, 7735, 7725, etc.
Hopefully one day I am able to purchase an Airport Monorail… until then I don’t plan for any news from TLG for a new Monorail theme. I am just happy they keep releasing (great) new train sets