Apple is having a product announcement event on October 30th. https://arstechnica.com/gadgets/2018/10/theres-more-in-the-making-apple-announces-october-30-event/
Ars doesn't mention iMacs, but does mention the possibility of an updated Mini, which may be a viable option as a replacement...
I looked at doing that a couple years ago, but I think this far down the road, I will tinker with it *after* I have my replacement, in an attempt to get some more use out of it. But not as my primary machine.
My 2009 iMac (which I bought refurb in 2011) looks to be at the end of its rope with a hard drive issue, probably. Shuts down when I try to boot in Safe Mode. Booting from my bootable backup for now. The question is whether to go out and get a replacement now, or wait to see if a new iMac is...