My general tip: Slow the hell down.
This doesn't just mean what JarJar said, that you need to improve your general technique before moving onto more finished pieces (although he is 110% right), but when you are actually finishing pieces, you need to take it slooooow.
Seriously. You do not have a deadline. And as a beginner, you couldn't realistically work to a deadline, either. THAT IS OKAY. I am still a learner and I still take a long time to do what other artists can do much more intuitively and more quickly. It is totally fine to work slow, because you are not answerable to anyone right now.
It can sometimes be difficult to not just rush ahead and get something done for the sake of being done, but take your time.
First, I will suggest that you thumbnail ideas. Thumbnailing is super useful. If you're unfamiliar with the term, I'll explain; a thumbnail is a teeny tiny sketch that you use to plan out a finished image. Do them fast, no detail, just blocking out shapes and focal points.
These allow you to think of your composition really easily, since it's much easier to think of composition when a- you can take in the whole thing at once and b- when you're not distracted by details.
Second, with regards to composition, keep the
rule of thirds in mind. Don't stick to it rigidly, but keep it in mind. Usually you'll want to avoid symmetry. I'm noticing in that King of Trolls drawing that it's very symmetrical, and the castle crashers fanart is a bit, too. Try and avoid this, it makes the drawing look boring and it doesn't lead the eye anywhere.
Sometimes you'll *want* a fairly static image, but with those two I doubt it's the case.
Third, when colouring, you really, definitely need to slow down. Do some colour studies. Think of them like thumbnailing, but with regards to colour.
This is a much nicer guide to colour studies than I could write up.
Personally, I usually do a colour study once I've already finished up my linework. It psyches me up to get on with colouring and feels to me like a more natural way of doing things to me. Usually I find I hit a "What the hell am I DOING?" point when I start colouring if the colour study isn't fresh in my mind. Other people do a colour study as soon as they've finished thumbnailing. Whichever works for you, really.
In your colouring: again, slow down. Think about your light source. Are the shadows all consistent? Have you shaded your forms as though they're 3D, or do they just look embossed? I think you need to study real shading more; do some pencil drawings focussing on shading. Make sure to use a variety of pencils; my bare minimum when studying shading is a 2H, 2B and 4B. Softer pencils are a must because they are much darker than harder ones.
Trust me, doing more realistic shading will inform your cel-shading. It'll make it easier to place shadows if you can think of how real shadows work.
Last but not least,
here is a list of things to look at before you can call an image finished.