So here is a basic interpretation of what i mean. First off the perspective on the legs of all the bots in the foreground are completely confusing (i really dont care to touch on the anatomical topic that earlier posts had mentioned as im not one to comment on stylized anatomy unless it looks truly bad which isnt exactly the case here) So via the greenish lines, i hoped to sort of accentuate some of the differences between the current nearly 'isometric' perspective legs where the top face never seems to come towards the camera, but rather stay on its own flat plane and tries to twist its way downward without the luxury of a third dimension. let those forms bend outward so that they come towards the viewer. It definitely helps to establish these forms BEFORE even thinking about doing any sort of detailed pixel popping, or else you risk the sad reality of having to lose all that nice detail because the basic forms dont quite make sense
(did i mention that the shading work is really nice?
)
As for the other UFO forms, really round out those forms and make sure the geometry behind them is clean and symmetrical. theyre looking very WIP'ish as your upper caption would suggest. taking a short amount of time to even these forms out should greatly enhance the composition and strike a more immediate aesthetic chord within the viewer's mind. Shouldnt take more than several minutes to clean those ovular forms up, which is good news!
The orange strokes sort of imply to me how the composition of your piece should logically flow given all thats currently present in the image. it roughly follows this already, but the forms really break the flowing comprehension from element to element. try to keep in mind that flowing organic nature of the work while finishing out its composition for a maximum synergenic impact.
Lastly, the horizon was suggested to be more ovular by other members rather than just being flat, which is indicated by the blue line there. Id say that if this were a side scrolling game mockup, its current level nature would certainly suffice, but i can see a great benefit to the piece by trying to round that horizon a bit. it will echo / reinforce the ovular shapes pronounced by the ufo ships which is a good thing. Even if you decide to keep it flat however, i feel like there is a big issue with how the ground was rendered as a whole. it feels very cobble stone ish in nature, and much like the legs of the mechanical monsters, they seem to suffer a bit from a lack of perspective and form, regardless of how cleanly theyve been rendered. your stone slabs need to be much flatter and more compact as they reach the horizon line. Once again, if its a game mockup on a limited platform maybe this would be more acceptable, but for a study of composition this needs some serious address. Reason being, it is the one unifying element that you have in your composition that ties all the other ground elements together. if that large component doesnt convince the viewer that its accurate then it pretty much shatters the illusion of unification youre trying to achieve.
I can tell with this piece you are having a blast really focusing on rendering beauty into the individual elements, but without those basic forms and rules of perspective, all the beautiful rendering tends to suffer in the confusion to the eye. remember your audience do not need to be artists to be able to distinguish when things feel 'off' or 'wrong', even if they lack the artistic understanding to explain exactly why that is the case.
You mentioned that it would be a real pain to basically 'start over' and while i dont think you need to do that, it could certainly benefit from a lot of formation before detail before you proceed to introduce any further detail, and this might mean redoing a lot of things you currently have rendered.