Intel today officially announced its lineup of 10th-generation "Comet Lake" desktop processors, which includes a number of chips that would be appropriate for the iMac. AnandTech details all 32 of the new processors in organized tables, but only a handful of the chips would make their way into the iMac.
At the top of the list is the flagship Core i9-10900K that Intel calls the world's fastest gaming processor. The i9-10900K features 10 cores and a base frequency of 3.7 GHz, while Intel's Thermal Velocity Boost can push single-core speeds to 5.3 GHz. It's also the successor to the i9-9900K chip currently used in the highest-end iMac configurations, so it's likely the new chip will make its way into Apple's lineup.
The current iMac family, with the exception of the outdated low-end model, uses a combination of 8th- and 9th-generation "Coffee Lake" chips, all of which have natural replacements in the new Comet Lake generation.
Natural replacements for the 21.5-inch iMac lineup include the new Core i3-10100, Core i5-10500, and Core i7-10700, while the 27-inch iMacs would see the Core i5-10500, Core i5-10600, and Core i5-10600K, in addition to the high-end i9 option.
While these are reasonable guesses at what we might see in the next-generation iMac, there's no guarantee that Apple will go with direct successor chips at each spot in the lineup. That's particularly true since rumors are pointing to a "substantial" refresh of the lineup with a potential redesign, including a rumored 23-inch model.
Timing for the iMac update remains uncertain, as the 23-inch model rumor claimed the new machine will be coming in the second half of the year, while reliable leaker "CoinX" cryptically said in March that both iMac and Mac mini updates would be coming "soon." The Mac mini update did come two weeks later, but we've yet to see anything official about new iMacs.
Intel says the new 10th-generation Comet Lake desktop chips will be available at retail and in desktop machines starting in May.