"Add player" function for hardcode & lua
Note: Originally this ticket double-featured bugs with custom tag, but it's been brought to my attention that a recent build has fixed the issue. Title has been updated to reflect this.
Debugging certain multiplayer features requires the tester to have two buddies or two extra clients open, which is generally a lot of hassle. Adding dummy player nodes also means coders and scripters have an easy way to test AI in non-SP gametypes. While the value to this is self-evident, I'd add that AI would also be a valuable debugging tool for testing game mechanics which require more complex multiplayer interactions.
2.2.5 is right around the corner, but I'd strongly urge us to consider the value in making this a priority for 2.2.6. I don't know the full logistics behind the feature, but I already know we do something similar with the Tails bot, so I imagine it's quite feasible at least for offline play.