Thanks OP, for making this thread. For me, Prometheans as the biggest problem with the current Halo games and contributed significantly to my lack of enjoyment of playing Halo 5 (played it on my pal's XB1).
What's wrong the Prometheans:
1. Aesthetic Design:
Whilst the floating component, "Rayman-esque" aesthetic may look cool in concept art, when modeled in-game the design appears dull, generic and generally uninteresting. Much of the issues with them as an FPS enemy, in terms of their gameplay design, stems from their overall aesthetic design as mostly non-humanoid, pseudo-robotic, floaty-component, semi-intangible entities... they lack a sense of physicality in the game which affords a strong sense of visual feedback for the player. More serious, however, is that their sterile and mostly robotic aesthetic simply doesn't fit the tone of the Halo universe; at least from the perspective of someone who has no exposure to the EU content and has only played the games.
2. Mechanical Design:
The biggest sins of the Prometheans as FPS, and specifically Halo FPS enemies, can be seen with their mechanical design; i.e. how their artistic concept translates into gameplay mechanics.
Too many of the Prometheans exist in a form that is essentially non-humanoid, and thus limits their capacity to offer compelling gameplay opportunity or a decent challenge to the player.
You have the "doggy-type" ones, which both look lame and barely offer more than an annoyance to the player.
You have the floating "drone-type" ones, which again are pretty worthless and not really fun to fight at all.
Then you have the more "humanoid" Prometheans, which you would expect to be as dynamic and agile as the Elites, Brutes and other Covenant enemy types... well... you'd be mistaken, as these "humanoid" Prometheans are probably some of the worst enemies to fight in any FPS. The humanoid types aren't agile, they barely move around and even when they do it's mostly via teleportation (which feels a little like cheating to the player), they aren't by any measure intelligent, and for a game series that historically prided itself on great enemy AI design, Halo 4 & 5's Prometheans really dropped the ball massively.
What they need to do to make the Prometheans a compelling enemy?
Well, the aesthetic design is mostly set in stone now, however, they can at least riff upon the visual themes presented in previous games to try to make the Prometheans more visually appealing and have a stronger sense of physicality in the game.
- Visual feedback - I like the idea of adopting a "Binary Domain"-type system of hit feedback, where mechanical parts fall off the enemy as your shots continually impact on the exterior of the enemy and do increasing damage.
Adding to this, blood effects for the more organic parts of the enemy as you shoot them, would help to further reinforce their sense of physicality in the game.
- Smoother more natural animations - I'm unsure of whether it was by intent or a failure to achieve their goals, but the current Prometheans animations just don't convey any sense of weight or physical presence within the game-world. The effect is such that the movements of the Prometheans appear jerky and un-lifelike. In future games, 323i should focus on making the Prometheans motion appear more fluid and natural, conveying a sense of weight and physicality through their motions and reactions to their environment.
- More humanoid designs - They really should get rid of the "doggy", "drone" and "walking T-Rex anime mecha" forms of the Prometheans and replace them with more humanoid forms which can be animated more naturally, as well as offer more compelling combat opportunities through greater physical agility and physical mobility around the battlefield (instead of just cheaply teleporting around). They need to move away from enemies which are no more than "teleporting mobile turrets", "flying turrets" and "dog-mounted turrets", and move towards designs that make the player feel more like they are facing an army of elite soldiers with similar combat ability and mobility of your spartan allies.
In Halo 5, the Spartans were presented as being more mobile and agile than they ever had been in a Halo game, and yet, most of the game has you facing enemies which are probably one of the least agile and mobile that a Halo game has ever seen. It's little wonder many player didn't enjoy fighting them. The developers gave you all this added ability, and they faced you off against hordes of dogs, toy-helicopters and teleporting, man-sized metal gears. They were poorly thought out from the start.
On the other hand, no enemy is worse than the flood. The shooter game enemy archetype of "little shitty things that run towards you and explode" are categorically the worst idea ever in the history of shooter games. I hated it as the flood in Halo and I hated it as those little exploding bastards in Gears of War. Please devs... can we let this archetype die already? It's lazy and it's not fun... so yeah, whilst the Prometheans were lame, NOTHING is worse than the flood. So I strongly disagree with the OP on that one.