they release a new AD major revision more or less every year for a long time.. as explained above, normally it's not that big a difference.. A new software component maybe, or some new features for existing components. For a while they were ever doing releases on season (AD summer 09, AD winter 09? which was bound to be great fun for anyone in the wrong hemisphere)
Of course AD18 was a huge jump (similar to when they did AD10, and with a similar huge list of side effects and issues to iron out)
AD19 will just be a continuation of what's happening with 18, maybe with a new feature somewhere.. probably will be mostly just more bugfixes, the way AD18 is going so far.