LUCENE-5229 proposes different changes, some of them being controversial, but there is one of them that I really really like that consists in refactoring the Collector API in order to have a different Collector per segment.
The idea is, instead of having a single Collector object that needs to be able to take care of all segments, to have a top-level Collector:
and a per-AtomicReaderContext collector:
I think it makes the API clearer since it is now obious setScorer and acceptDocsOutOfOrder need to be called after setNextReader which is otherwise unclear.
It also makes things more flexible. For example, a collector could much more easily decide to use different strategies on different segments. In particular, it makes the early-termination collector much cleaner since it can return different atomic collectors implementations depending on whether the current segment is sorted or not.
Even if we have lots of collectors all over the place, we could make it easier to migrate by having a Collector that would implement both Collector and AtomicCollector, return this in setNextReader and make current concrete Collector implementations extend this class instead of directly extending Collector.