I don't think they'll introduce an AS iMac Pro for two reasons: (1) The original iMac Pro was intended only as a stopgap, to keep pros satisfied until they introduced the 2019 Mac Pro. (2) The functionality of an AS iMac Pro, which would offer Max and Ultra processors, is already available with the Studio plus the ASD.
However, a large (consumer-priced) AS iMac does make sense, since they offered that before, it was popular, and there is nothing avaialable now that replaces it—you'd need an M# Pro Mini and a consumer-priced 27" Retina display, neither of which Apple offers.