Wasn't it the plan to remove these interfaces in 3.0?
We could deprecate Fieldable in complete and leave it as it is.From Lucene 3.0 on we only have AbstractField. So the old Fieldable interface must be used internally until 3.0 (with the deprecated methods), but user-land code like Solr should only overwrite AbstractField and not implement the interface anymore (I am not really sure, why Solr needs this Fieldable implementation at all, it the only place in Solr where problems occur, it would be good to reimplement this internal class using AbstractField).
Document.add() and all other public appearences of Fieldable should be overloaded with AbstractField counterparts and so on, so that all public API only use the abstract class anymore.
But thats my opinion, and maybe is related to the other issue "whole document/field reimplementation".