Ok, here comes some CC!
The posing of the characters is kinda weird. Practice around with different posing to figure out what looks good and what doesn't.
Other than the torch light, it's pitch black. Try giving the scene some night lighting.
The sword on your character is positioned on his wrist.
The person on the left is holding his bow wrong.
The torch on the left is emitting no light.
The portal itself is just one big scaled portal block. In the library, you can choose to repeat the block, so it's the right scale and it looks normal.
Flatlands
The camera angle is kind of weak. What I usually do is position the camera close to the ground and make it look up. Maybe add a little dutch angle? That's only what I do. You don't have to copy of course. (Remember, this is cc, not hate like swift thinks)
That's kind of it! Follow community CC and see what others post and how they make it, and you'll get better before you know it! Good job!
Swift or Slime please don't downvote, I'm begging you.