I would look at getting a refurbished iMac directly from Apple to save a bit of money. Most of the Macs in my office are refurbished and they run without issues – Apple gives you the same 1-year warranty (here in North America) as a new Mac and I believe you can get AppleCare (expensive) if you want an extended warranty.
In my opinion, some of the current Apple computers are handicapped due to Apple's refresh cycle or the manufacturing constraints of their suppliers – e.g., the AMD Radeon Pro 570 and 580X are actually old product and quite a way behind the state of the art, especially at the prices Apple is charging. The Vega is an expensive option and at this point it is also an old architecture. The Radeon Pro 5300M and 5500M in the 16" MacBook Pro are two generations newer (going from AMD Polaris to Navi) and perform like you would expect midrange mobile graphic processors to do in 2020.
Check out AMD's upcoming 5600M and 5700M's specs to see what the new iMac GPUs might look like: https://en.wikipedia.org/wiki/Radeon_RX_5000_series#Mobile
So, in my opinion, it would be a good idea to wait until the refresh, or buy a current or used iMac at a decent discount if possible, because they aren't worth what Apple is charging.
Intel's current 10th-gen (Comet Lake) CPUs aren't a huge jump from 9th-gen, but Intel added more cores in the flagship CPUs and the Core i5-10600 has 12 threads rather than 6 because Intel brought hyperthreading back (your performance mileage may vary) as well as increased cache, going from 9MB to 12MB. There is a good chance that Apple will also include Wi-Fi 6, which will benefit you in the long run.
Regarding heat – if the newer processors can get the work done faster, they'll spend less time at max power. If you're doing a long gaming session, however, you'd want minimal throttling and excellent heat management without too much noise – we'll have to see if the new iMacs can do that. I think they will definitely have a lot more graphic power than the current iMacs, but at a price.
The other possibility is that Apple enlarges the display in the current 21" iMac chassis to the rumored 23", which might give you a middle ground choice. The 21" iMac has typically been a better value than the 27", but Apple has always handicapped it with lesser hardware. That being said, the 27" 5K displays in the current iMacs are fantastic and they do make the computers seem like a better deal compared to most PC monitors.