My Luan uses Lucene as its database, but uses an old version because everything that is maintained by modern programmers only gets worse over time. While the core idea of Lucene is very good, modern programmers have been steadily making Lucene worse and more complicated.
There is no good reactionary database that I know of. Lucene seems like the best starting point. The two options are forking Lucene or rewriting it. Either way, this is badly needed because any serious application needs a database. I would be glad to help in any way that I can if someone would take this project.
One thing that Lucene lacks is a write-ahead log for durability. I have implemented a write-ahead log myself on top of Lucene here but this really should be built directly into Lucene.