Reindexing (at least with Zebra) leads to search not working for some duration of time, depending on the size of the catalogue. So often you want to do reindexing at night, when use of Koha is low. But remembering to start the indexing process before you go to bed at night is error prone. We could have a syspref called something like "DoReindexInTheMiddleOfTheNight", and a cronjob that runs "in the middle of the night", and checks the syspref. - If it has been set to "on", a reindex is triggered and the syspref is reset to "off". - If it is set to "off", nothing happens.
I like the idea to make this somewhat available to libraries. Would also be very interesting for Elasticsearch where the users can change indexing in the GUI.
I don't think we'd want library users to have that kind of power. Reindexing a large collection can take a really long time.