I really, really recommending making a beta-version available to minimize errors in the final product.
EDIT Revising some stuff
Total Warfare and general stuff:
-Buildings (TW pg 167), urban combat is a fact of life.
-Scenario creating guidelines.
-Standard rules relevant for scenarios, such as carrying and lifting stuff, hidden units.
-Make sure to enable these to be used without a GM, assume smart, co-operative players.
-Basic linked scenarios, "Campaigns" along with XP points for MechWarriors.
-Clan Honor should be included but perhaps include a more clear, game-like form too?
-All equipment rules classed as Tournament Legal or Advanced in 3150 or later. There are also several experimental technologies that would be nice to have (such as RISC APDS, Interface Cockpit, Void Signature System, and other such used in some XTROs), on the other hand, prototype systems are irrelevant (eg Clan-tech prototypes, Age of War of prototypes).
-(Armed) Industrial 'Mechs should be included as an option, especially for those playing the early Dark Age sub-era.
-Keep art limited, minimalistic. No full page or half-page pictures that have no purpose beyond looking cool. Art style should be coherent. Art should never bump important information to another page or interrupt the flow of information.
-Rule addition: allow flamers to deal both damage and heat at once?
Tactical Operations:
-Planetary Conditions (TO pg 28-68).
*Exclude extreme depths and bug storms as too odd and/or specialized.
*Simplify fire and smoke.
*At very least include some of the additional terrain types and time of day and weather modifiers.
-Advanced Determining Critical Hits, Expanded Critical Damage (TO pg 75-76 [and the relevant tables, of course]).
-Called Shots (TO pg 78).
-Extreme Range (TO pg 85).
-Expanded Reversing (Flipping) Arms (TO pg 87).
-Other Combat Weapons and Equipment (TO pg 99-104), everything in the section.
-Rerouting Heat Sink Coolant and Heat Sink Coolant Failure (TO pg 105).
-Artillery (TO 179-186), simplified?
-Minefields (TO 207-210), include ability to pre-place them?
Strategic Operations:
-Simplified customization (SO pg 188->) rules for players to use if they use experience gaining and keep existing pilots between scenarios. Maybe no rolling dice required but instruct players to establish what level of field refits is allowed.
-Design Quirks, include any added in TROs etc. Logically part of the construction rules. (SO pg 193-199)
Interstellar Operations and related:
-Direct Neural Interface. (IO pg 68)
-SLDF Advanced Neurohelmet. (IO pg 68-69)
-Clan Enhanced Imaging system. (IO pg 75)
-Possibly simplified Manei Domini for Jihad era games like in Jihad: Blake Secrets.
Absolutely no:
-Strategic Operations stuff except for customization which i've described above. This would be clutter in a book that is more or less a basic 'Mech rules book.
-Campaign stuff beyond simple linked scenarios arguably can be found in Strategic Operations and Campaign Operations. Clutter for 'Mech rules.
-No BattleForce, Alpha Strike. This isn't about quick-playing system or large scale stuff.
-No short stories beyond perhaps opening fiction. Unnecessary. Small fluff side-bars are acceptable and perhaps even recommended, they're what sold me on BT long ago after all.
Unsure if exclude or include:
-LAMs and QuadVees. Their nature as hybrids makes them questionable for a book about 'Mechs since they rely on vehicle and aerospace rules. On the other hand, they are 'Mechs and their special rules are mostly expansions to 'Mech rules. And LAMs transforming into fighter-mode can be treated as leaving the gaming area and counting as destroyed/retreated units.
-Super Heavy 'Mechs are just too specialized and rare, as cool as they are. On the other hand, they're but slight expansions to normal 'Mech rules.
Might have other ideas later.
EDIT Added a couple of points.
EDIT Another couple of points, from Interstellar Operations.
EDIT Removed irrelevant stuff i have no real opinion about.