Brought up by Anjew in a thread to have production buildings such as barracks and war factories to set their stance of any type. This would allow newly created units to have their new default stance set to this.
With more buildings of the same type, it would be one stance for all these buildings?
Yes, this is a good idea. I think it'd be best to be able to assign a production stance to individual buildings. E.g. in TD you may want one barracks to produce infantry on defend and another on attack move. It's trivial to double click to select all barracks / WFs / etc. and the same stance could then be applied to all.
How about having a global option (in settings, perhaps with a configurable hotkey too) to change all production stances. That way, players choose between their preferred stance without having to change it every time they make a new production facility / every game.
The most sensible place to manage this is on the production queue. That means that it is set per-factory in TD, but per-type (infantry/vehicle/etc) in the other mods. In RA there is no guarantee over which structure a unit will be produced at, and it would be awkward to have units pop out with a different stance just because you placed a new structure immediately before a unit finished construction, or because a stray harvester drives past your main war factory at the wrong time.
This isn't a difficult feature, and so while i'm not a huge fan of the scope creep this seems like a good compromise to make for the next playtest. I'll take this on.
A part of me is kind of against this but at the same time i feel like i could live with it. One of those things you need to just try and get a feel for IMO.
Slightly leaning towards one default stance for created units then having the player micro manage new units.
Would be nice if this https://github.com/OpenRA/OpenRA/issues/11727 is being added too.
All of our mods have at least one actor that defaults to the HoldFire stance (submarines, spies, stealth tanks, stealth raiders, etc), and having per-actor defaults is fundamentally incompatible with per-factory stances. The two can't coexist without inventing an unreasonably complicated UI that would immediately defeat the purpose and benefits of the feature.
I personally think that moving the stance choice to the factory is in line with the goals of the stance changes (replacing long-standing hacks around missing features with proper UI), and the best solution to this non-problem, but I refuse to go anywhere near such a controversial change after the disgusting behaviour I have had to deal with from parts of the community in the last couple of weeks.
Removing from the milestone and my future plans.
I pushed a WIP branch to https://github.com/pchote/OpenRA/tree/production-stances if anybody wants to pick this up, and will probably file a PR for the *Init plumbing changes since those are more generally useful.
Idea compatible with your WIP:

When you build different units, you might prefer to keep builtin default stances, but when you are training many Rifle and Rockets and executing some tactic, you could change their default stance optionally and temporarily.
Probably clear - that checkbox in production queue is to enable/disable custom default stances.
Working on it
Any update on this ?
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This isn't a difficult feature, and so while i'm not a huge fan of the scope creep this seems like a good compromise to make for the next playtest. I'll take this on.