I don't understand what the current iMac doesn't do that a future new iMac might do that's making this decision so difficult for you that you feel forced to wait. If you need an iMac now, get one now. It isn't going to suddenly stop working when the iMac line is updated.
While true, I'm looking at a configuration in the range or upwards of $3500. I'm not spending this kind of money on hardware that is approaching 2 years in age. This should be a computer for years to come and it's not like Apple is currently asking a lower price with the new iMac on the horizon in 2019.
I know, these days CPU advancements have slowed down, but I'd still like to use the current generation of chips, particularly since Intel suddenly has to compete and offer more cores due to AMD's recent resurgance. Furthermore I'm expecting major changes to the iMac design.