Yes, guys, sometimes they come back!
Fourteen years ago Vania Kashperuk created a class which became classics: SysMultiTableLookup. It made possible to create lookups based on multiple tables. Some people are still looking for it in vain among the standard classes. So I decided to create a new version of it for D365FO.
It combines supporting for both lookup and lookupReference methods: performFormLookup and performFormReferenceLookup.
Vania added a tutorial form to his class, so did I. There is a form in the project, which shows how unbounded form controls can be looked up with fields and display methods from joined tables as well as the case with aggregation and a CustTable data source to demonstrate a lookup for a reference group.
Take note that I did not implement resolving and have not tested it thoroughly, neither.
Feel free to use it and elaborate at your own risk.
The whole project tmxSysMultiTableLookup is here in https://github.com/wojzeh/tmxSystemTools
The only class is here