This is similar to, but not quite exactly, https://tracker.moodle.org/browse/MDL-47318.
Scenario: most classes at a school meet once per week. For these classes a weekly format is just fine and includes the benefits of automatic current topic highlighting. But some classes are intensives - like three-week long classes that meet each day. And some classes are hybrids, with one week of intense activity on campus and the remainder of the class online stretched out over several weeks but not a full semester. In these cases the topics format is the best choice.
Problem: instructors are not very good at manually indicating the current topic.
Proposed solution: an (optional) ability to associate a given topic with a given date range when editing the topic summary in a course that uses the topic format. I imagine something like a checkbox labeled "assign topic to date range" that, when checked, would reveal a couple of fields from which the user could indicate a start- and end-date. When the current date falls within that range, that topic would be highlighted as current. Presumably, then, the Quicklinks plugin could also be update to take advantage of that so it would work with both weekly and topics formatted courses.
Notes: topics with overlapping dates need to be dealt with gracefully (or prevented). I second Hartmut's suggestion that the lowest numbered topic be the default when there is more than one possibility for current topic (again, see https://tracker.moodle.org/browse/MDL-47318).
But also note: this is not the same thing as restrict access; the topic can always be available, I just want a way to automatically mark it as current.