searchlib which is at present only used in mod_forum and report_configlog is quite old and relatively unused but conceptually a very useful and powerful concept.
It could be used in many more places, but in particular as a layer on top of Report Builder (
MDL-70343) and possibly also in global search.
I think many people would agree the gold standard here is Jira's JQL:
Building on report builder would be the most natural way forward, in theory anything that exposes all the column metadata to enable generic filtering can be gracefully translated into a query language. For the existing uses of searchlib it probably makes more sense for them to be migrated into report builder rather than directly improving searchlib itself.
It might be too much of an ask for the expression language to be able to be translated into different domains such as into both sql for report builder and also into for example solr / elastic search query language, but in theory if done well this should be possible.