Just to come to your attention. Halo: Reach is not all that bad.
These are the factors needed for halo 4 from Reach:
*Forge - Works perfectly right now. It just needs more themes, props, maps and maybe utilities.
*Armoury - The purchasing and customisation on your armour outdoes halo 3’s armour by far. Maybe add some halo 3 armour as well as keeping reach armour.
*Gametype Customisation - Keep the insane gametypes and options ‘n’ all. The only change required is bring back VIP and more custom power-up types. Also add a scepator team who are on white team and stay in theatre mode and zombie loadouts.
*Loadouts - I can’t see the problem with them. In matchmaking, they could limit it to just 2 or 3 loadouts. At least keep it in custom games for the creativity and stuff.
*Invasion - Fun gametype and all. People just got bored because they were limited to the amount of maps. Just add community maps and different traits etc.
*Firefight - Worked fine and could be customised massively. They should add earn-able credits on default maps and “pro” gametypes and up the players to 8.(4 a team) Btw, keep the target locater, plasma launcher, grenade launcher and maybe dmr, needle rifle and concussion rifle.
*Matchmaking - It still works. The arena functions well too. Just a few rank modifications.
*Challenges and commendation - works brilliantly and gives a sense of achievement. Commendations should also address to specific weaponry as well.
*Ranking - This should be split into two systems. EXPERIENCE and SKILL. Experience is the halo: reach credit system based more on rating number. Skill is based on a combination of halo 3’s highest skill system and average reach rating system (arena rating). In other terms:
EXPERIENCE - shows how experienced you are and the game and how much effort and time you spend on it. This will be earned by game ratings, performance, victory, commendations, challenges and jackpots. Credits can be spent of more things to do with your armour and other unlocks.
SKILL - Based on how good you are at the game. Uses a combination of halo 3’s highest skill system and altered with reach’s rating system. Progress should be viewed after matches. Allows you to get to higher ranks, have a medal next to your rank (onyx in arena etc) and other major upgrades.
*Armour Abilities - I’m guessing that everyone is gonna rage at this, but I would like to keep them. Matchmaking should give out mainly just jetpack and sprint (or non) while custom gametypes should supply all of them (+ a few more!)
*Campaign - Don’t forget the sabre space fights!
*Bloom - Should only be implemented on close combat weapons like the assault rifle or while dual wielding/in a first person view in a vehicle.
So yeah. There are a few things that Reach did well on and should be carried on to the next halo. 
The Arena system does not work, quite simply the original ranking system based on individual ratings, before it became W/L, was seriously flawed for obvious reasons (and why Microsoft have always had the TrueSkill system based on team effort to avoid the problems which you probably saw during individual-based Arena ranking {there’s an article about this, try google}). Arena is mostly quantity over quality, why punish skilled player’s in favour of people who can play 24/7?