Check out my feature request for the same idea: Manage Docker on Another Host
High availability is unlikely to ever become a feature. It is very niche.
However, sustainability and allowing users to expand their system without discarding their weaker hardware is a more realistic goal.
EDIT: My point with sustainability is that we can rightfully encourage any user to start with a Home Assistant Green knowing that they can always add additional hardware and their Green will not suddenly become a useless paperweight. Essentially, when you outgrow the Green, move your addons to another system and keep Home Assistant Core on the Green.