As someone who's used Lucene and Solr extensively, my biggest wishlist item has been support for upgrades. Typically Lucene (and Solr, and ES) indexes cannot be upgraded to new versions (it is possible in some cases, but let's ignore that for convenience). For many large projects, reindexing is a very expensive (and sometimes impossible) ordeal.

There are cases where this will probably never be possible (fields with lossy indexing where the datatype's indexing algorithm changed), but in many cases all the information is there, and it would be really nice if such indexes could be identified and upgraded.