This has been discussed at length ever since the Pi0 was launched.
The RPF/RPT categorically do NOT discuss future products. If an upgrade is in the works, you'll find out when it is launched, and not before. The reason for the Pi0 was to see how close to free they could make (and sell) an SBC. That makes it extremely unlikely that an upgrade will happen until the price can be held to the same $5.
There are some serious constraints on upgrading the Pi0. Mainly, finding a better SoC that is compatible with other Pis, can use PoP RAM, and is cheap enough not to blow the BoM costs.