Add scripting capability
The idea is to learn a good lesson from ElasticSearch and add and promote quick scripting options. Not everyone is ready to deep dive into java code or step with a debugger through core Solr code, that they didn't implement or may don't even understand properly.
Scripting could allow for all sorts of "crazy" stuff, like changing runtime order of analyzers or search components, affecting on scoring, introducing advertisement entries, calculating some analytics, refining facets etc etc. The field (once done properly) could be infinite. This can help evolve the code base faster and actually get a feedback from larger mass of people out there, using solr.
One way could (possibly) be ruby / scala / some other JVM friendly language that is able to load stuff from java packages. I'm sure some custom Solr scripting engine can be more efficient too.