That got me thinking, it seems that forerunner weapons and the prometheans themselves are basically of the same type of construct. Could it be possible that a Promethean is basically just a weapon with a soul?
I have two ideas of my own, anyhow. I'll try and draw up some concept art, but first lets look at what I've got.
Promethean Chimera
Around the size of a wraith but with much thinner armor and much faster. The front end fans out like a ghost, but the back end is heavier, akin to the Revenant. You enter through the back, like a banshee, and the cockpit forms around you as you lay down on it. No hanging out. Anyways, it forms when activated, like all Forerunner artifacts. I'm thinking that the jets on either side are not there at first, but are piles of vertical metal pieces that raise, and then triangulate into a sort of tube shape, albeit with gaps in traditional Forerunner tech style. It should move a little slower than the revenant did, because of it's bulk. It's just barely faster than a warthog. It can fire it's weaker weapons while boosting, which consist of a set of three suppressor-like weapons at the front that spin around while fired like a revolving machine gun. It is also taller than it is long, and so there are two seats as it goes up, one for a gunner, and another to operate a turret. This opposes the horizontal lineup of the riders in a Warthog. The turret will be a long range variant of the scattershot, albeit weaker. Think of it as a gauss cannon with a different shot texture, spread, and a damage output somewhere around 3 or 4 carbine shots.
Promethean Valkyrie
The Valkyrie is a small two seater jetplane that can fire it's weaker weapon while moving, like the Chimera. In appearance, it looks sort of like an upside down Banshee, but without the elongated shield at the front. Instead, there is a cockpit that hangs under the wings, and the gun is above. The weapon should fire similarly to the binary rifle, having a long range shot, but with high recoil. The damage output will be significantly lower. A gunner can join you. The secondary weapon can not used while mobile and you must stop. The Valkyrie can hang in the air like the falcon, or move like a banshee, but these are it's only two modes of mobility. Overall, the movement is similar to the UNSC Falcon, where it is more concentrated on where you aim, especially when turning. You can also move straight up and down at the cost of this mobility. While you are in one place you can fire a weapon that shoots an incineration cannon that does not multiply, similar to the function of the banshee's fuel rod. However, it is shot in an arc like the concussion rifle. The Valkyrie is great for quick fly by tactics while you snipe your enemy with it's laser, weakening them for ground troops to finish off. You will need to learn how to strategically place yourself, fire off the powerful cannon, and then sweep for stragglers to make the best use of it.