Most of the helmets have a rubber ribbed section in the inside as seen below
This image the section seems to extend past the jaw area, like a motorcycle jaw guard.
This one seems to stay in the neck region.
Besides that, the inside of the helmet is actually just a texture. There is no interior.
However, if you have a visor, the interior of the helmet should technically have a frame inside it that holds the visor, with the opening (barrier, rim, whatever) running around the same rough area as it is on the exterior of the helmet. Like as seen in Halo ODST
Really besides the jaw, the interior is entirely up to you.
Another thing is the little mechanical parts seen in halo 4 on the outer rim of the visor. I think they're supposed to act as projectors for the HUD.
Here's Halo 5's concept for chiefs