How powerful do you see these maneuvers being (particularly when it comes to encroaching on the BM's remit) and how long do you see such training taking?
First thing I want to address is that, like I said, BMs will be better than anyone else at using these, if I go the route I'm currently thinking, because they'll be able to treat them as extra manuevers, and use them as an attack even if they use an action for everyone else, and add their superiority die to the damage (you wouldn't normally get a damage buff for most of them).
Secondly, I do want to point out that I'm only doing that because it makes sense to me, I don't actually care about the Battlmaster's niche protection. It's a single subclass, it's not important.
As to their power and training time, Kobold Press' Beyond Damage Dice has stuff like this;
"
Pinning Shot. When you make an attack with a bow, you may attempt to pin a Large or smaller creature to a wall or surface by catching their clothing with the arrow’s point. This maneuver can be used against creatures without clothing at the GM’s discretion. Make an attack roll; if the attack hits, the target must make a Strength saving throw. On a failure, the target’s speed is reduced to 0 until the arrow is removed. The target can make another Strength saving throw as an action on its turn."
And that seems fair to me. It's situational enough that it shouldn't be an action, but I also might consider making stuff like this require a bonus action to "aim" or "focus" in order to do as an attack, otherwise it's an action. Idk, depends on playtesting probably.
Spending several months training with a master before moving on to find another sounds like the optional training to level rule.
Months would be long enough to make most folks not even want to use the system, IME. Better to keep the time to a week at most, unless the technique is more powerful than average. I use a "train to level" system in my homebrew game, but that's because it doesn't have level packages like dnd, instead using levels only to gate certain things and make challenge building easier on the GM. So, you can learn new tricks either by spending character points for traits, or by forgoing the normal benefits and opportunities of an Extended Rest to spend that time training. Basically you either train to level, or gaining training as treasure. I don't like trying to do that with dnd levels, though, because they're so abstract that it's best, IMO, to just lean into the abstraction.
Also part of the point of this idea is to allow playing out the fantasy of learning individual techniques from masters, developing your own, teaching them to others, etc, which doesn't work for me or my players when it's just flavor thrown on the leveling mechanics.
Picking up a specific special technique in a few days without dedicating yourself to martial endeavours (multiclassing) sounds like a good justification of taking a feat.
Well, you would have to have proficiency in order to even train to learn the technique, so "without dedicating yourself to martial endeavors" part doesn't really apply, here.
It these are just available to be gained without any character advancement investment, then boons would be one option. Another would be to treat them like the way a wizard can gain extra spells for their spellbook. - Finding a master willing to teach can be made easy or rare like scrolls as treasure, and most would require paying, or equivalent service for their training.
Yeah, I really think I might have some greater techniques that are like a boon, having to earn them as if they were treasure, but the normal ones are like a ritual with ritual caster, you just find someone who knows it, or a manual that contains it, and spend appropriate time and sometimes money to learn it.
If a wizard with a longsword can just pick up a maneuver that doesn't require superiority dice, that is really going to cheapen the BM raison d'etre however. - I'd regard that as a boon more powerful than a feat, and fighter players would probably look upon it like a wizard player finding that fighters could train similarly to cast counterspell 1/day would.
I think that comparison is wildly inaccurate. "Doesn't require superiority dice" ignores that if you Trip someone (already a thing anyone can do) without superiority dice, you do it in place of an attack, and don't get to add a die to anything while doing it.
It's nothing like a fighter getting a 3rd level spell (much less one of the most powerful 3rd level spells) for free once a day.
Not to mention that most fighters aren't BMs.
I do think maneuvers are the best chassis for this, it’s just a question of how often you want the technique to be used. You could provide 1 superiority dice an encounter for example but just for that technique.
I really think that leaving out the dice, and making them at-will but only situationally as good as just attacking, should work fine. No damage bonus, just special attacks, expanding on the special attacks in the DMG.
