Regarding lessened focus on mech stuff, I think a lot of the reason people don't play with those forces is because they aren't even perceived as that important, or useful.
Battletech was invented as a game of big stompy robots, and that's been the continual focus. Infantry and tanks were added as options, but whenever the question comes up why vehicles are so easy to disable the answer from TPTB is because they specifically want mechs to be the kings of the battlefield, so even the best vehicle has to have a downside by sole virtue of not being a mech. The art also shows this focus, non-mech units get very little representation.
That means, if you REALLY want other unit types to get a fair shake, you'd have to convince CGL to make BT into a game of future armored combat where we have giant robots instead of just big stompy robots: the game. That means less squishy conventional arms, and that means more prominent depictions of stuff like mechs getting swarmed to death by a squad of infantry while a demolisher shoots it in the face, and the quintessential mech leading the charge with a bunch of toads hanging off.
This extends to the games as well. A LOT of people come here because of the various video games, and non-mech forces have even worse representation there. People get into the tabletop game because the video game was fun, and vehicles rarely factor in to that fun while infantry and aerospace pretty much never do.
Honestly, if there were to be a reformat, the biggest thing happens to always be new equipment. I wonder if equipment guides, by era, wouldn't work as the format for rules expansions.
Having stuff solely sorted by era would be really confusing when you have units sporting equipment that debuted in three different eras. You shouldn't have to figure out what era something is from before being able to find rules for its use.
Personally, if we are discussing a reorganization of rule books, I definitely think everything should be split between gameplay rules and construction rules, so that means splitting SO and IO as well.
For an actual redo of the line, though, my preference would be to add a Battlemech construction guide to supplement the BMM with rules and equipment for building just mechs.
Then add a Combined Arms Manual and Combined Arms Construction books covering the gameplay and construction of vehicles and infantry, and any equipment they use that mechs don't have.
Add an Aerospace Manual and a Construction book covering ASFs, Dropships, and anything else in space along with the space combat rules plus rules for aersospace units on ground maps and ground units in space at the back. It's probably best for any equipment aerospace units use that's not in the mech books get rules even if they are duplicated in the combined arms books, just to reduce the confusion of having to flip through your entire library to cross reference every single book (basically, the mech books are the core rulebooks everyone needs, and additional unit types only need their book and the core mech book).
The last book should be the campaign book combining the high level maintenance, support and logistics stuff from SO and IO.
Battleforce I see as being more or less superseded by Alpha Strike unless they wanted to fix up strategic Battleforce a bit.
On a side note, equipment cards would be a nice supplement.
