Good Question OP, To many seem blinded with sheer hatred or love with either style of the Halo franchise to ask what was liked about both we could keep into the future, your question is one that many should be asking.
Personally, This is the blend I would love to see.
Halo 2:
High base move speed
Choose between Elite or Spartan
1 grenade starts on MP
Fantastic maps
The traits of the BR
Hijacking (Not that it’s going anywhere)
Good ranking system
Clan system
Beautiful soundtrack
Split campaign (I know alot disliked it, but the suspense of knowing what happened to Chief after his fall into the ocean on delta halo was a great feeling)
Halo 3:
Amazing custom game fun
Perfect aim assist balance
Menu Music (gives me good chills every time I hear it)
Brute Chopper
The Scorpion in Halo 3 was my favourite, it was faster than the models we currently see, but less powerful explosion.
Vehicle strength and damage ratios
A mix of the smaller big team maps in 4v4 social.
2 shot Fuel Rod GUN (was its Halo 3 name)
VIP
Territories
Halo 3 ODST:
Cool action adventure campaign for Rookie (be nice if spartan ops in Halo 5 was like this)
Max 3 types of each grenade (seems the most balanced, some like 2, some like 4)
Grunts would Flee sometimes, but less, because you were not a spartan.
Best Firefight system (not customization)
Halo Reach:
Plasma Repeater
Plasma Launcher
Revenant (Would like to see multiple variants, like the warthog)
Weapon sounds (the sniper rifle sounded BEAST)
Customization System (alongside the ranking system link to my Idea for it: https://forums.halowaypoint.com/yaf_postsm2779917_Halo-5-Ranking-System--A-great-way-to-go.aspx#post2779917)
Firefight customization (link to my idea https://forums.halowaypoint.com/yaf_postsm2788389_New-Firefight-Variant-Now-Has-Spoiler-Tags.aspx#post2788389)
Invasion
Headhunter
Stockpile
Race
All other gametypes
Custom searching options for MP
The amours looked Awesome
Precision editing in forge
5 shot kill magnum
PP stength
the needler in Halo Reach is the best IMO
Halo 4:
Be able to choose starting weapons (for the Carbine, Storm Rifle and Magnum, need a new pistol for the covenant that can be a select-able side arm)
Weapon/Armour skins
Magnets in forge (need to be less glitchy)
Lighting in forge (needs to be something you can choose to add to a map)
A powerful Carbine
Pulse grenades (need a buff)
Dominion
That is the stuff I would like to see carried over, stuff I would like to drop:
Sprint
Starting AAs
Perks
DMR, LR, PP, PG, BS in loadouts
OP Beam Rifle
High Bullet Magnetism
And some other stuff I cannot think of just now.
That’s what I want, now maybe anyone who posts next can actually do what I did?