I think that each weapon should have an adjustable scale and selection boxes allowing us to augment individual weapons for CUSTOM GAMES. For the sake of example I will be using the M6(Pistol/Magnum) series as my example. The following in blue is a list of known M6 models taken from Halopedia:
M6A Handgun - Used mostly by civilian security forces, standard issue/black polymer finish.
M6B Handgun - Used mostly by civilian security forces, KFA-2 x2scope, hard chrome finish.
<mark>M6C Magnum Sidearm - Halo 2</mark>-era UNSC sidearm, ‘up-sized’, armed with High Penetration ammunition, black polymer finish.

<mark>M6C/SOCOM - Halo 3: ODST</mark>-era UNSC sidearm, issued with smart-linked 4X scope VnSLS/V 6E, sound suppressor, muzzle brake, and black polymer finish. Nicknamed “auto-mag”.
<mark>M6D Magnum Sidearm - Halo: Combat Evolved/Halo: Combat Evolved Anniversary</mark>-era UNSC sidearm, ‘up-sized’, armed with High Explosive ammunition and featuring KFA-2 x2 scope.
M6E Handgun: standard issue/electroless nickel finish.
M6F Handgun: smart-linked scope KFA-2/electroless nickel finish.
<mark>M6G Magnum Sidearm - Halo 3/Halo: Reach</mark>-era UNSC sidearm, ‘up-sized’, armed with High Explosive ammunition and electroless nickel finish.
<mark>M6H Handgun - Halo 4</mark>-era UNSC sidearm, ‘up-sized’, smart-linked scope KFA-2/electroless nickel finish.
M6I Handgun: select-fire variant; detachable shoulder-stock/black polymer finish.
M6J Carbine - UNSC Army carbine variant of the M6 series. 355mm barrel, detachable shoulder-stock/electroless nickel finish.
M6K Handgun: police ‘undercover’ variant; 6 round magazine capacity/black polymer finish.
As you can see there is a lot we have had had, and a lot that has only been mentioned in the fiction. People always complain that their favorite variant of a weapon is gone and the reality is all the weapons seem to change in almost every iteration. More recently however, with the Anniversary Playlist, they changed Reach’s M6G so it would emulate CEA’s M6D. A grand idea that I would love to see taken to the next level.
First off, changing the weapon behavior. If we were to select a weapon on the ground I believe we should be able to adjust the following: Rate of Fire, Damage, Accuracy, Bloom, Reload Speed, Magazine Capacity, Total Capacity, sound pitch (and other sound aspects I don’t understand), and I’m sure other things I’ve not thought of.
Second, I would want the ability to have certain add-ons that appeared in past Halo games. These selection boxes should include: Smart Link Scopes, Suppressor (Silencer), Flashlight, dual wieldable (if mechanic is available in future game), and perhaps some other things that I can’t think of off hand. Finally, I would like to see the physical weapons from past games make an appearance, different from each other only in looks, but each pre-set to most accurately imitate the behavior each weapon had in their original game.