When I worked at UOForever, I modified the core in such a way that the era was dependent on the facet you were in.
I'm not sure they ever fully used the system, but it basically meant that your character could go from a UOR facet to an AOS facet, then to a TOL facet and the game mechanics would change to suit. You'd have one bank-box and one backpack per era, which were switched out when your era changed so that you couldn't take era-specific items from one to the other. Some items could be flagged as able to Phase between eras and therefore would come with you.
With this kind of system, comes a lot of work, even things like Mining and Fishing must be modified to yield different results per facet/era, etc. Every Core.AOS, Core.UOR, etc, check needed to be changed to somehow check against a facet's era support.
I know the system isn't the same as what OP's describing, but I wanted to describe it to show there are other ways to achieve waht they want without having to wait months to release the next expansion. I feel like starting with UOR or AOS and releasing successive era's after that as OP suggests, would end up making previous content more and more redundant. Items acquired during AOS will be useless in ML when people get hold of major artis and such. I know that it's the natural development of the game over time, but I disagree with artificially recreating that successive expansion trend. Might as well just go straight to TOL and fill in as much as possible, as soon as possible. Truth be told, I don't think there's enough time left on UO's life to not just use TOL out of the box.
