Very rough study idea for revising Crawler heads. I think a big part of the disorientation is that everything we've seen Forerunners use that are mechanical nature (meaning actual Forerunners get a pass) is that they usually only have a singular eye or node or whatever at the center of their frame, and then 4 introduces us to all sorts of two-eyed machines. They also made the mandibles utilize very basic geometric shapes instead of the intricate "layered" bodies of Forerunner enemies. In this concept, the "eye" would double as the actual point of fire. Really, Forerunner sentries shouldn't even need eyes given the vast array of security/observation systems on Requiem that could stream them any combat data they needed. Anyway, the bottom lists a few possible ideas for heads.
Basic Unit (Boltshot) - Rounded head, permanently closed "mandibles." Maybe one or two floating parts off to the side.
CQC Unit (Boltshot Alt. Fire) - Similar to Basic Unit, but the mandibles will open up and glow brightly, warning the player that the Crawler is charging a shot. These types are also more tackle/charge-friendly.
Alpha Unit (Suppressor) - bottom-right illustration shows a general idea. More floating, glowing pieces permanently in place around the head as well as multiple "eyes." This would also be the justification for the higher rate of fire, as more nodes = more bullets.
Sniper Unit (Binary Rifle) - See bottom-right and bottom-mid for a basic example. This one would have much more protection around the head which would open up into an almost Promethean Knight-styled headdress, which would glow a deep crimson revealing it was about to fire a Binary Rifle shot. Pieces could always move or flutter around the head following a shot or to possibly indicate a warning.
Rather than add all sorts of ridiculous pieces onto the Crawler, using colors would make more sense given existing Forerunner motifs. You could just do something simple like Bronze - Silver - Gold for Promethean ranks, or change the colors of the bodies/weaponry. Go Red - Orange - Yellow - Blue - White or something. Star colors.
