This is how and why a Halo 2 Remake needs to happen:
The why, simple:
Fans want it
There is money in remaking it
It would complete the Forunner Saga (Halo Reach-Halo 3) in HD on the Xbox 360
To compete with COD and EA (BF3 and MoH) pumping out shooters every year, 343i and Saber Interactive can release Halo 2 HD in 2013, NLT 2014. It doesn't have to be on the 12th or 15th year aniv. of Halo.
Make it 39.99 like Halo CE Aniv., which was a great price for you got in return.
How they should make it:
1. Just like Halo Combat Evolve Aniversary (Halo CE Aniv.). Have Saber Interactive and 343 Industries (343i) put an HD cover of the original engine for Halo 2.
2. Just like Halo CE Aniv. have 3D enabled for gamers/fans that like 3D HD gaming.
3. If possible create the final mission where Master Chief is on "The Ark" and ends up falling into the atmosphere for the Beggining of Halo 3. You already have a template for this level in the Halo: Uprising Novel and just need Saber Interactive to use the engine for the game that Bungie designed and make the level (have 4-8 checkpoint parts in the level and like 1-2 Five to Ten min. cutscene. Then you can cut into the Gravemind/Cortana cutscene. It can be released with the Halo 2 HD remake in 2013/2014 or later in the form of DLC if it takes long to develop, create, and be ready for fans to play. Bungie stated that they always wanted to have that level in the game (fans too, with us all a little dissapointed with the cliffhanger ending). This would make Halo 2 complete and the way it should have been. Cause in my opinion, Halo 2 would have been the best Halo ever if it didn't have that cliffhanger ending. I mean it is the best and most known multiplayer game of all time (me and friends still take about Halo 2 multiplayer and campaign and how it was what got us to buy an xbox and play the original Halo CE.
4. Like Halo CE Aniv. include a map pack for classic Halo CE (PC version as well), Halo 2, and Halo 3 maps. Make them for Halo 4. This would make fans/online gamers/MLG players come back and play Halo 4 multiplayer, increasing the longevity of Halo 4. Brings something fresh to players new and old. Especially if you have 4-5 of these maps:
Lockout
Midship
Beaver Creek (know it was remade in Reach but a great map)
Hang Em High (same as above)
Elongation
Colossus
Ascension (same as above)
Ivory Tower (same as above)
Foundation
Warlock
Sanctuary (same as above)
Headlong (same as above)
Zanzibar
Guardian
Narrows
The Pit
Snowbound
Burial Mounds
District
Uplift
Also if 343i was smart, they would make a subscription service like Battlefield 3 Premium, Call of Duty Elite, Bungie Pro, etc. and release a remake of a classic make for free to subscribers every 1-3 months to keep Halo 4-6 multiplayer fresh and trips down memory lane. 343i could release alongside the new maps that subscribers get by paying for the service.
4. Like Halo CE Aniv. you need to have achievements. Fans got them in Halo 2 Vista, but lets be honest, H2 Vista was crap and had so many errors with playing on Windows Vista and even installing the game on current generation Computers and Laptops because they made it on Vista.
Also to promote fans/gamers to actually play the campaign other than for trips down memory lane and gamerscore, make playing Halo 2 HD remake, Halo CE Aniv., Halo 3, Halo 3 ODST, and Halo Reach campaign/multiplayer achievements unlock armor/weapon parts/skin variations for Halo 4-6 campaign.multiplater. Like if you completed Halo: Reach - Halo 3 you unlock this weapon skin(s) and Armor piece(s). It would make the replay value higher for the campaigns of all the games as well the multiplayer of Halo 3/Halo 3: ODST, and Halo: Reach because they can unlock armor/weapon variations from unlocking those achievements. Kinda like what Halo Waypoint did with the Police and CQB helmet, body, knee, and shoulder armor unlocks in Halo: Reach.
5. Create Terminal/Data Pads/Audiologs/etc. in Halo 2 for players to find. The skulls in Halo 2 were one of the funnest and challenging thing to do alone or with a friend. It added something new to the campaign and added some replay value to see if players could make it through the levels with added difficulty. All 343i has to do with the skulls is make them useable after players collect on them on any level like Halo CE Aniv. and Halo 3. Adding Terminals/Data Pads/Audio Logs/etc. for players to find in the campaign can give 343i an opportunity to tell another layer of the Halo Universe for fans (such as maybe giving hints to where the other Spartan IIs and Spartan IIIs went to during the events of Halo: Ghosts of Onyx as an example, especially because those events happened b/w Halo 2/3 and leads in Halo: Grasslands and Kilo Five which lead into Halo 4. Make finding the Skulls and Terminals/data pads/audio logs/etc. an achievement(s) like Halo 3, Halo 3: ODST, Halo CE Aniv.
6. Co-Op Campaign over Xbox Live like Halo CE Aniv. is a must and possible because of the system link capability as well as online mulitiplayer capability of Halo 2.
These ideas would satisfy every dream a Halo fan could have for Halo 2. HD remake, achievements, and the lost level of Master Chief on "The Ark" would have fanboys heads exploiding as well as campaign fans. The classic map pack included in the game would excite multiplayer fans/MLG players because most of the maps I suggested are symetrical and have been favorites for competitive play, especially Midship, Lockout, Hang Em High, Beaver Creek, and Warlock. The map pack would bring players back to playing Halo 4 multiplayer because they have something new (and old) to play.