TG/DB are by design DLC, and hidden behind a paywall. They have design elements that spread over Tamriel (unique objects and dark hand targets), which were also designed as a finished list - aside from thieves chests.

Similar to this, psijic breaches and excavation sites were also designed to spread around Tamriel, but with a more random placement and infinite grind in mind.

It would make sense to add dailies unique object thievery and dark hand targets into the DLC zones that came after TG/DB, that would be limited effort and an easy way to extend these. Adding new heist/assassination zones would probably require much more effort for a very limited return for ZoS.

A Morag Tong/Camorra Tong (Morrowind anniversary ?) event would also make sense, giving us illegal tasks all around Tamriel, which woud be independent from TG/DB but with same taste.

It's also possible HB and GC would become free access and part of base game, such as IC some time ago, which would solve the paywall restriction, and allow to build on these skill lines more widely. TG/DB have always been part of TES games after all...