Respectfully, gameplay features are important but there is more to player numbers than simply gameplay features. The appeal of the setting for the game, medieval for Age of Empires II, is important. There have been many RTS games released over the years but medieval, modern warfare and science fiction themed RTS games are most popular. Promotion of the game is also important. Microsoft has the resources and opportunities to promote their games many times over than which is available to other companies.
As far as the adding more features to the game, I feel that this is a slippery slope. The definitive edition of the game should be just that, definitive. The truest edition of the game compared to the original game release to run on modern hardware. I had hoped back in 2013 that HD would have satisfied that itch but unfortunately there wasn’t anything like the Age of Empires Insider Program back then to help make that edition of the game an overwhelming success for players of all types.
For helping new players to the game, instead of QoL features, I would like to see better integration between the William Wallace tutorial campaign and the Art of War missions such as a statement at the end of the tutorial campaign encouraging new players to try the Art of War missions next. I would also like to see more Art of War missions offered as has already been excellently suggested here on the forums.