Seasonal format is a cool way to experiment some more wacky stuff but the biggest flaw is how it is counterproductive or counterintuitive to the typical desire of long term investment in mmorpgs. Please bring some type of carryover between seasons, if there will be another season (I think it’s always a good option even alongside a permanent classic+). The simplest option is that in the next season, for each character you level to 60, you can are allowed to boost/transfer one of your characters from the previous season with some sort of indicator like a cosmetic that they were a SoD character. They can not bring any of the gear, just give them some generic weakish lvl 60 gear. Potentially could bring some of the stuff like mounts or maybe not.
In this case, SoM players would be able to boost a lvl 60 character of the same class/race from SoM into SoD with entry level 60 gear and some type of SoM cosmetic. Either only once per account or once for each character they level to 60 naturally in the current season. Add some type of long term progression or carryover to seasons if we continue with this idea.