Well that could work out
The only way to balance out the mammoth is if the covenant had an equally powerful unit and I can't think of one that matches the overpowered-ness of the mammoth, considering it can one shot a huge as forerunner turret, let alone a scarab. The covenant do have equally powerful units, Scarabs.
So so it probably won't be in game, despite how cool it is
Mammoths would have a much more powerful attack but weaker armour
Mammoths would be faster in a sprint, but less agile/manoeuvrable
Scarabs could climb buildings/terrain, mammoths couldn't
Scrabs could also literally glass the beams of the Mammoth pretty easily. The mammoth simply doesn't have the armor to block out Scarab beams, especially if it's something that of a super scarab, or a way bigger scarab.
Plus, the Mammoth is essentially a Cobra in a lot of ways. Just a Mammoth has a way, way more powerful gun, weaker armor, and it also takes like 5 minutes for it to reload and be shot again. It's similar to the Covenant plasma turret on one of the multiplayer 1v1 maps in Halo Wars 1. Just you couldn't move it, and it had a infinite range essentially (you could literally shoot the enemy's base with it without even seeing it)
But all in all, I just don't see the Mammoth being in Halo Wars 2, simply due to the way it is. Could possibly 1 shot enemy team units pretty quickly, and deal a ton of damage to enemy bases.