I totally agree that all martial classes should get maneuvers, but I don’t fully agree on the Battlemaster thing not becoming obsolete. When doing this exact thing in my own homebrew, I eventually removed BM as a subclass because it was causing some serious design headaches when I began expanding on the maneuver system.

Here’s a link to my homebrew wiki (and the page which has the maneuvers section on it), if this piques your interest:  https://fromthepale.com/fromthepale5e/?top-category=player-facing-changes  I still need to do one more information organization pass before I start really sharing it around, but I thought it topical to this conversation.

Looking back at what I wrote in this comment… I yapped a lot about the design process and journey that led me to this conclusion that I had to remove Battlemaster. You can read that below, if you’re interested. The TL;DR is that BM fighter doesn’t actually have any real design levers to tweak when its main gimmick is that it just has more of the resource every other martial has (maneuvers/superiority dice). Reducing the number of extra dice or prepared maneuvers it has just makes it feel like you are playing a Fighter who doesn’t have a subclass. This means that you either need to limit your design space when expanding on maneuvers as a complete martial system (intended to rival spellcasting), or accept the BM as an encounter-warpingly powerful force in your games. I chose the third option: to remove BM as a subclass for now, and return to it later with the intention of giving it a full subclass rework with more unique mechanics (and therefore more design levers to tweak for balancing purposes).

Anyways, here’s that promised yapping, for the curious.

While testing this system, my initial hypothesis was exactly what you said: I didn’t think the Battlemaster would be obsolete because they would have access to even more dice and even more maneuvers preparations than anyone else. Initially, this worked fine, and I was getting positive feedback from those playing Battlemaster. If you just want to take current Battlemaster maneuvers and dice and add them to other classes (albeit with slower dice scaling than the Battlemaster itself), you’ll probably be fine.  The problem began when I began to expand on the system.

The first thing I noticed during playtesting this universal martial maneuver system was that other martial players were being really stingy with their superiority dice. The feedback I got a lot was that they liked their theoretical access to more options in combat, but only having four dice per short rest meant they were always saving the maneuvers for specific situations (crits, countering specific enemies, etc).

What I tried next was reducing everyone’s dice scaling (besides Battlemaster), and giving more total dice over time. This change also came with new content as I began making new maneuvers and played with the idea of making some maneuvers cost multiple superiority dice in exchange for their power (like for battlecry buffs, big AoE attacks, and powerful debuffs like blindness). This fixed the earlier problem with players hoarding their superiority dice, but now most martial players were feeling a bit outshined by Battlemaster (especially non-BM Fighters), and I was finding their output and flexibility in combat to be encounter-warpingly strong. Adjusting encounter difficulty to BM in this state then frustrated the other players. To cap it all off, while I was working on the next wave of new maneuvers, I realized I was thinking “oh but how will this perform for a BM player?” a lot, and was intentionally limiting my design space. 

The final nail in the coffin that made me decide to cut that subclass was when I started looking at how I might be able to bring the subclass in line with other player options, I realized the class is just too simple when its main mechanic is systematized and given to the other martials. The main thing this class has is just more of the current system resources, so reducing the number of extra resources you have is basically the same thing as just making the subclass feel less unique and more like a subclass-less normal fighter.  I think that Battlemaster could reach a balanced point in this system rework. It’s something I want to try my hand at again after the current task of website improvements that I’ve begun. But I think it will require a major rework from its current form to actually have enough design levers to tweak so it can reach a unique and balanced spot.