Current Lucene and Solr indexes implement quite a no. of features according to their specific APIs, design and implementation. However in the long run, while differences in APIs and implementations will / can of course stay, the difference in design can make it hard to keep those features on par.
It'd be therefore nice to make it possible to abstract as much of design and implementation bits as possible in an abstract full text implementation which Lucene and Solr would extend according to their specifics.
An example advantage of this is that index time aggregation will be implemented only once and therefore any bugfixes and improvements in that area will be done in the abstract implementation rather than having to do that in two places.