How would they be balanced with other weapons?
Here’s my idea
Spiker: I think that if the Spiker was added into the game, I think that it should become a loadout weapon. I think that it would have to be the strongest automatic (besides the SAW and Needler) because of it’s bullet drop and slow bullet speed.
Brute Shot: Basically, make it like the brute shot from Halo 2: bounceable off of walls, 4 shots, maybe add more shots to make it more of a power weapon, on par with the Concussion Rifle. It would not be in loadouts, of course.
Spike Grenade/Firebomb grenade: This is a tough one. I think that MAYBE these grenades could be added to loadouts. While I don’t have any ideas for the Spike grenade, I have an idea for the Firebomb. Make the firebomb more of a support weapon. It would’nt do much damage, but its burning affect would last for a long time, so that it would be great for getting assists, or for finishing off a weak enemy and then making a get-away.
I also think that maybe it would be possible to add a precision brute weapon to the loadouts, but I am completely in the dark about how to make it work.
Make sure to add your thougts/ideas.
As much as I want to see them return, they would be too redundant. Most of the weapons in this game are redundant as is.
The DMR and Lightrifle might as well be the same weapon.
The BR and Carbine might as well be the same weapon.
The AR and Storm Rifle might as well be the same weapon.
List goes on.
We got the Hammer at least 
Spike Rifle would be the same as the Halo Reach iteration. However it would be a secondary weapon along with pistol, boltshot, and plasma pistol. It’s perk though is that it provides slightly more powerful melee than normal at about %110 power.
Mauler would become a secondary weapon and it would fill a similar role as the UNSC pistol, it has a 2x zoom, has 14 shots but it burst fires two shots, and it is a 6 burst shot kill. It’s secondary perk is the 2x zoom.
The Brute shot is an ordnance weapon and to differentiate it from the Concussion rifle it has 4 grenade shots per clip. It can bounce off walls, and pull some of the tricks in Halo 2. Also it’s melee has the same melee damage modifier as the spike rifle.
Gravity hammer duh.
The spike grenade serves the some role as the plasma grenade only it excels at infantry damage. Same as the Halo 3 iteration, you can only have one per loadout unless you have grenadier.
The Flame grenade is the same as the Halo 3 iteration only it is only ordinance due to it being so powerful.