The ability to create a thesaurus of synonyms is invaluable, and exists in on-premises SharePoint but not SPO, this should be remedied.

Customers use the Search Thesaurus feature for many scenarios: to map acronyms to their expanded terms, to provide relevant results when companies merge or are acquired, to map corporate vocabularies to "plain English," and many more.

This would also make it possible to allow "Other Labels" including alternate languages in a Term Set to also be treated as synonyms in search results, which IIRC is the way it works on-premises but it also seems we've lost the ability in SPO.

These gaps not remedied by Graph or Delve, and the inconsistent UX between on-premises and SPO is a potential show-stopper for some enterprise migrations. Though some have suggested workarounds using Query Rules or client-side scripts to embed expanded synonyms in search results, query rules do not scale for maintenance or performance, and the existing client-side solutions are neither complete nor supported. We need a proper remedy to bring SharePoint Online search back up to par with both the on-premises version and the competition.