What I've always been interested in is give the ability to extend the lists.
At the moment there is a hook there, but its use is quite limited (in fact it can just be used to add/remove things in the surroundings of the list (i.e. above/below), because columns and rows still depend on the function retrieving the data from the database.
TBH I'm not sure how to extend (efficiently) this part.
Of course one option could be to hook the functions giving the possibility to extend the queries, though that's a "difficult" way (i.e. it requires passing pieces of queries around that may not be that good), another option would be to add another hook able to "inject" elements directly into the array here (but it implies some more processing and most likely yet another query per hook, that could be "annoying"). Well, in that last case could be "enough" to allow multiple $this->_listOptions['get_items']['function'], without having to rely on another hook (and include the processing into the class itself.

Are we aiming at something like that?
Did I miss any other obvious option?

Another thing I did in another branch (that I don't remember if I pushed or not, for sure I didn't PR it) was to split the template in a way that it was possible to render the "body" of the table and the "additional_rows" separately, allowing to re-use at least one of the two templates when needed (this because certain times may be useful to maintain all the flexibility of the original template, but changing "deeply" the body for example, and in the current situation the only solution is to duplicate everything.

P.S.
I know this is out of the scope of this PR, but while we are discussing my beloved createList... 👼