You bring up a lot of interesting ideas and discussion (as do the subsequent posts). As lead sandbox developer for CMT's SPV3 for Halo CE (https://www.youtube.com/watch?v=_TQSGzR95xU), it's interesting to see your take on all these weapons, as we have had them co-exisitng for quite some time in both campaign and MP settings, with very similar roles as to what you have described. So I figured I'd drop in and offer my insight with the experience of having all these weapons in one game, as I've deeply analyzed the roles these weapons have had in prior games.

AR: As people said, this weapon is at it's best in H4 and is a viable alternative to any of the precision weapons. HOWEVER this is also the first time it's also been an ideal CQ combat weapon since H1, as H3's was expected to be used in conjunction with grenades and melees... and Reach's was severely underpowered in both campaign and MP settings (unless against an elite in MP... given the larger body target from my experience in invasion at least. Where Reach's did excel though was it's burst given nature in campaign where the reticule bloom encouraged you to fire in bursts at mid range before moving in for the kill or using it in conjunction with a headshot weapon. This is the role I have it in for our project as well, where it is effective, but not ideal for CQ combat due to a lower ROF and shallow clip. Finally as you suggested, we let the display function as a digital zoom, giving it a hint of ODST SMG behavior, to encourage more ranged combat. To the person wanting a grenade launcher on the AR, well we actually have that too as a special loadout only weapon at the start of a few missions.

SMG: Being a big H1 fan, this is how I have factored in the SMG into gameplay. It's the ideal automatic human weapon for CQB, with a large clip (largest in the series other than the LMG in H4 which IMO is just a glorified AR), high ROF, and quick reload, ready and melee animations. This is in contrast to H4 where the AR also has these attributes so I would imagine including this in H5 would likely also result in the AR being downgraded. This would fall in line with the more "classic" take on gameplay they seem to be going towards, where the AR wouldn't be as viable an option against headshot rifle wielders.

BR: This is an interesting one, as I also have had to balance this against against the pistol, DMR, and our Particle Carbine (next generation take on the carbine, using head and no ammo). We've done this through 2 ways. One is giving it bullet physics like H1's pistol and the H3 BR, instead of always being "hitscan" like the DMR and Sniper. The other was by revisiting Bungie's
original intentions for it, before seemingly bending to make it function more like the H1 pistol with it being a "zoom and fire several headshots" type gameplay. If you remember, originally it was only meant to kill in 4 bursts at close to mid range, with the zoom behavior firing single shots and needing 12 hits total to kill someone. Obviously, 4 shotting is at the heart of Halo's combat since H2, so I am presuming the loss of it's scope in the pictures we've seen is an attempt to preserve it's behavior of 4 shotting while opening up ranged combat to other weapons, as the BR once again dominates the precision rifles in H4 since the patch. Personally, I prefer single shots when zoomed as it feels awesome to cut down grunts like that (which is how we have it set defaultly when picked up, although we do allow you to toggle the zoomed burst on and off for SP) but perhaps 343 feels like that is to big a departure for the weapon.

DMR: This is a much more enjoyable weapon I find in H4 than reach, given the lower bloom, and being made into the primary human weapon with a scope as it would seem now, it can be more effective at range than the BR but less effective at closer ranges. Which I guess works. What we've done for ours is give it the hitscan ability and bring its autoaim to the same level as that of the snipers (although it does have it unzoomed unlike the sniper) making it more effective at range than even the single shots on the BR, while preventing it from being as powerful at close to mid range as the BR.

Pistol: This is one I think H4 nailed perfectly, our take on it is a bit more in line with H1's when zoomed, and more like Reach's quickly firing but highly inaccurate behavior when unzoomed if the player chooses too, but I don't see the Pistol changing much at all for H5.

Carbine: H4 also I feel nailed this weapon with it's lower damage and high ROF. In many ways, it's a lot like the ODST pistol, so I don't see this one changing much as well. Our take on it uses heat rather than ammo, and does lower damage as well bringing it even closer to the ODST pistol.

Hope this insight into our weapon behaviors gives you an idea of how I think this will all work out in H5, it's quite funny how all the ideas seem to follow the same basic ideas and concepts despite all 3 of us (CMT, the people here, and 343) having little interaction with each other.