Spartan Customisation has evolved nicely over the Halo Games, from changing colour to attachments and more varied choices in armour pieces.
Personally though, I think customising Spartans shouldn’t be just cosmetic. It should evolve in to a way to truly personalise your playstyle etc on certain playlists.
I suggest allowing us to tweak the our Spartans functionally. We could have a series of stats such as strength, speed and dexterity and link them with a points system. As you add and take points away from each set of Stats, your Spartan gains some slight advantages and disadvantages, not enough to be game breaking, but enough to leave some thought on how to plan your fights and how you will fare in certain situations.
For example, you could make your Spartan faster, allowing you to get to a weapon quicker than your opponent, say a rocket launcher, your opponent arrives just moments after you do and sees you with the rocket, you may fire both rockets, but that Spartan may have a slightly better jump height and shields, allowing them to just deflect the worst of the damage. You may have a poor dexterity, so you can’t reload or swap weapons as quickly, so you retreat using your speed. Different combinations can give you slight, healthy advantage over your opponent and a slight disadvantage at the same time.
There also be a preferred weapon for a Spartan, for example, one Spartan may excel with a BR, whilst another would prefer a Sniper Rifle. This can also give a slight edge.
Players will also be able to tell what stats each spartan has in the lobby and also when they track down that players GT in game.
This would hopefully bring more varied and stregetic gameplay. Of course, it would only be available in certain playlists, so all the equal, skilled players who want their simple, beloved arena games can enjoy classic gameplay.