The title summarizes it, but let me explain.
I am just going to call it “Halo: Upbringing”, though I have the same topic in the poll forum named as “Halo: Ultimate”. I decided to call it something different. 
But it would be mainly online. It would combine all the halo’s, and it would have a ranking system, like levels 0 to like 100.
But wait, lets make this more interesting.
There would be characters from the different halo’s.
Halo 1’s Spartan, with health/shield.
Halo 2’s Spartan, with shield/regenerating health.
Halo 3’s Spartan, with its shield/regenerating health.
Halo 3’s ODST, with its shield/health. I call the adrenaline from the ODST a shield, because that is what I think it is…
Halo Reach’s Spartan, with its shields/health and the reticle bloom, so guns become more innacurate. Both the ODST and the Reach Spartan, and the reach elite are forced to have the reticle bloom, the other spartans do not get decreased accuracy. The final tier elite would have the bloom too.
From the low levels, you would start out as Halo 3’s Spartan.
Once you level up enough, the game FORCES you to play as Halo 2’s Spartan. It also locks out the Halo 3 spartan, and that spartan no longer becomes playable.
Once you reach a high enough level, you then get forced to play as the Halo 1 Spartan.
After that once you get a higher level, you then unlock both the ODST and the Halo Reach Spartan at the same time, and can switch them off or whatever.
Basically, the low levels would be more powerful than the high levels, and as you got a higher level, you just get weaker.
And, this is just the UNSC side of it.
I havent really thought of the covenant side much. But instead of going through the Halo’s one by one or whatever, I’d say just the toughness of the elite would be similar to their rank like for example.
The elite white ULTRA seen in Halo 2, is in my opinion, the toughest elite.
The gold elite is pretty tough.
And I’m sure other suggestions can be thrown in.
The Elite Ultra, and Gold elite though, wouldnt be in multiplayer, but maybe somehow could be incorporated for some weird game types like elites vs spartans or something.
anyway though, lets get back to the levels for the covenant.
low levels get an elite, that its health regenerates, and shields regenerate similar to the halo 3 elite, and the halo reach elite combined, and would match the halo 3 spartan.
The next higher level elite, has slightly less shields. But still regenerates health, and would match the halo 2 spartan.
The next higher level elite, has same shields as elite below it, but does not regenerate health as quickly. Still a bit stronger than a halo reach spartan.
The final tier of elite, that is the weakest, would have shields/health similar to the halo reach spartan. it still would regenerate health though, just cause elites do that.
Also having reached the final tier of unlocks, you would then unlock the jackal, and the grunt. Just for balance, they have a pretty weak shield, similar to the ODST. Just so you cant just one shot them in the head. They also get a helmet. So it takes 2 headshots to kill them. One shot to take off the helmet, the other to finish them, only though if their shields are collapsed. (One shot to the head with a sniper rifle).
The grunt, could also have stronger grunts unlocked before reaching the final tier. But I dont know how to balance them or whatever, and I am lazy, so yeah. But the final tier grunt, would be similar to a ODST in health/shields, and they will be the same speed too. Able to jump the same, and whatnot as well.
The Jackal, could also have stronger counterparts unlocked earlier before reaching the final tier, but whatever, yeah I’m lazy. The final tier would have the classic jackal shield. Since jackals cannot throw grenades, the left trigger would control the shield. As for how it is controlled, I do not know, perhaps forces the shield to cover the whole body, and when covered, movement is reduced, but you’re still able to fire from the cover. The screen changes to the color of the shield, because it covers your face too. The shield is collapse-able to enough enemy fire, and covenant weapons collapse it easier. One of the buttons, would also allow you to kind of do an “evade” move, similar to halo reach’s elite evade roll. It would be similar, but yet have a longer recharge, and would be their way of dodging grenades, like they do in campaign. Grunts would have a similar evade move.
Jackals also would have the classic jackal sprint, as seen in halo campaigns. Grunts can also sprint. ODST’s can sprint too.
I’d probably better be clear about this now. The classic armour abilities in Halo reach, would only be able to be used by the Halo Reach Spartan, The final tier Elite.
I am considering giving everyone else sprint, and only sprint. Everything else would also be slower than the reach spartan and final tier elite sprinting/evading.
A Halo 1, Halo 2, or Halo 3 spartan sprinting, are not faster than the Reach spartan.
Same with the elites. Only the final tier elite can keep up with the reach spartan, but all elites get the evade ability. Sprinting and evading take a lot longer to recharge, but only recharge normally if its the final tier elite. Same with sprinting. Also, because I sense it might happen, if you club while sprinting, the club does not do as much damage as a normal club. This is to prevent double clubbing someone to death, as I think that is lame to sprint up to someone, club them once, then again. Same with evading.
This is also a bit out of order, as I’m making stuff up as I’m typing, but lets talk about the levels, starting with 0 to level 100.
When you reach level 100, you have the option to start over again. But when you reach level 100, you have the option to reset back to level 0. But perhaps you unlock something cool for the next time you reach level 95 to 100.
The covenant can perhaps unlock a hunter class. Or even they might be able to unlock the white elite ultra.
The UNSC can perhaps unlock the Halo 3 spartan at level 95 to 100. (levels could vary)
The experience system would be similar to Halo Reach, with credits earned for random stuff.
Now, that you have read all of the above. Everything is not set in stone. But think about it, you play a 4v4, or a big team battle, and there are different spartans EVERYWHERE. Some elites, a couple grunts and a jackal.
There could be playlists where everything is available.
Playlists where its always elites vs spartans. Similar to halo reach’s invasion, or maybe just CTF.
A playlist where its only grunts and jackals against ODST’s.
A playlist where its Hunters and White elites Against the best spartan classes.
This is only limited by your imagination. And I could type lots and lots more. But I will not do it unless this topic receives views, messages, replies, and overall popularity. Then I can try to balance everything, explain more things, but until then, I do not need to waste my time typing for nothing. But you have the general Idea.