Promethean leader.(This is it Chief.)Phaetons as core Air, (upgrade: Changes rapid fire cannon to continuous beam. Second upgrade: Adds hardlight pods that seek air units.)Make the Cryptum (the sphere you mentioned) a Super Unit akin to the Condor & Scarab. Also the Cryptum isn't what converts humans into Prometheans; that's the Composer. I recommend making the Composer a unique building/base defense for the Promethean Leader. There's also already a concept for a scrapped Forerunner Tank from Halo 2 so you'd only need to add a few minor Promethean details to it.
The little flying Promethean VTOLs would be the support air, Y ability would be to attach to units and add a shield to block projectiles. (Upgrade: It can now heal others while being attached to completely different units.)
Warden Eternal or the Didact as a hero,
Preferably Warden. His basic would be the triple beam shot from his head, uses his sword up close and his Y would be the massive sphere he charges and fires from Halo 5.
(Y also comes with badass, "this is the end" sort of voice line when firing.)
Knights as anti vehicle infantry (Upgrade: Instead of firing one blast, they fire 5 shot lower damage bursts.)
Soldiers as core infantry, (Upgrade: Suppressor now fires explosive rounds. Second upgrade: They can occasionally Dodge incoming fire with their teleports.)
Soldiers with light rifles as anti infantry. (Upgrade: Changes light rifles to the forerunner beam sniper. Etc)
The pack of dog like creatures, (Name forgotten.)
would be the scouts/rush unit, like the jackrabbits.
(Upgrade: Adds one or two to the pack, etc)
Warthog like unit could be a small hovering vehicle with the explosive hardlight turret hovering on the back. (Upgrade: Faster and adds soldier clinging on to the side with Suppressor.) The soldier with the suppressor is also affected by the explosive bullet upgrade.
Maybe some sort of concept for a Promethean tank, maybe reverse engineered from UNSC or covenant designs with added Promethean tech and weaponry.
Maybe leader power is that massive sphere in halo 4 that kills people and converts them into Prometheans. Kills a certain amount of infantry and retrains them as a random assortment of soldiers, dogs and, knights.
Another leader power would be teleport with the usual very blinding space-time warp they do all the time.
Once again, another leader power could be a guardian warping in and applying a shield buff to a large area, or maybe something offensive, idk.
Another leader power would be a swarm of the little healing VTOLs flying around in a circle healing and applying a weak shield to every unit inside.
(You cannot add a shield to any unit who Is moved into the area after it has already been cast. However, it will still heal.)
Additions or changes are greatly appreciated, reply away!
Also the VTOLs & dog-like creatures you mentioned are called Watchers & Crawlers respectively.
Actually, yesss, thank you for the clearing of my mistakes with the cryptum and composer, but I actually agree.
Though, what would be the attacks for such a (seemingly) crazy strong flying structue?
And what would the composer research, build? OH, wait, that could be the leader building for the forerunners!
Thanks for the reply, Beastgun!