It could be a forerunner scarab style boss. So I’m thinking it would be round like a disk shape, with six legs all around it and it has two giant turrets that look like eyes, so basically it looks something like a giant robotic crab or something. The two giant turrets are like those promethean turrets that charge up and shoot the beam at you except they’re way more powerful and you can’t destroy them like the halo 4 turrets.
You have to destroy 3 of the legs to make it come down to the ground, similar to the scarab. And it has a small entry door, and there is a little hallway to the center with a small armory (kind of a reward for being patient enough to shoot all the legs 
and from there a winding indoors hallway to the upper level. At the top the is the main reactor, and above it there is a hole, the exhaust for the reactor so if you jump on top with a mongoose (like you could onto the scarab), you drop down the exhaust and you can take out the reactor right away.
So once you shoot up the reactor, the alarm goes off, and you have 30-45 seconds to walk back down the winding hallway and jump out. And just for clarification, this thing is huge, which is why I described the indoor passage ways as “hallways”, but it isn’t too huge. And another feature is if you get too close to it’s legs it will step on you.
So, would you like to see this in halo 5. In my opinion, it would add a little bit of variation instead of just having the scarab.
(p.s. The scarab would still be there.)