Now, im sure a whole lot of people loved the playable elites in halo 3,. i admit, i was one of them. but looking at halo infinite, i think playable brutes would be amazing as well.
halo infinite is going to change a lot of what 343 has been doing with the "ahem" less than 5 star entries. they are definitely changing it for the better, why not add playable brutes, playable Chraigs if you will,
If it was already in the cards, then playable Brutes would be an interesting new feature. However, as Porcin3 Proph3t
said in another reply, the cost of development would likely make this feature out-of-scope for - at the very least - the initial launch.
That said, character customization is extremely important, both for the longevity of a title and likely monetization strategies for its free-to-play aspects. I hope the trend of deep customization options (e.g. left vs right shoulder, helmet effects, etc.) continues with Infinite. Rewards based on rank/XP, seasonal events, and achievements could bring those in-game moments of shock into the everyday experience: "I can't believe they have flaming Recon."
I enjoy the idea of multiple playable species, but I would much prefer deep customization options of Spartans than have that potentially sacrificed.
TL;DR: Depth Over Breadth :)