Nobody knows. For me, the fact that the 27" got a decent update, but the 21" was unchanged (except for ditching the HDD) is an indication that we will probably first see a 24" AS iMac, and it probably will be one of the first AS Macs to be launched.
However, these kinds of indications are sometimes completely inaccurate. It reminds me of the 2015 MacBook Air. Everyone expected a big update in 2016, but the only thing they did was to move from 4GB to 8GB RAM. In 2017 yet again a big update was expected, but we only got a tiny processor bump, and it wasn't even a new generation (still Broadwell). The actual new MacBook Air came in 2018, but at a much higher price point, and they kept selling the old Air. The old Air was discontinued in 2019, but only in 2020 the new Air would have the same price point as the old one used to have…
Since then, I hesitate to give predictions without solid rumours.