Dancing Hoskuld
Deity
The problem here is that there are no bonuses they could provide, and we can't add bonuses freely still I am told. Having these improvements in a context where they do NOT provide any bonus (just increase tile productivity) would be a bit confusing I think.
There are improvements that go on a feature and don't provide bonuses. The forest reserve in standard BtS and my beacon and lighthouse improvements for reefs (which aren't working right) are examples. The lumber mill and watermill don't in std BtS but the lumber mill does in C2C.