Markus-
Nice work, glad you found some of my process stuff interesting. I think you're coming along great with these thumbnails with a variety of flavors and topics. The last 3 scifi ones are your best out of the batch. I think the first image and the second image are great explorations but they still need some sort of scale reference and focal point. I feel like that 2nd image is real close to telling a story but at this point it's missing the final knockout punch and some clarity. Is it a giant space station just floating there, is it being destroyed, etc.
I marked a few quick points on the 3rd image which to me you had more of the elements necessary to tell a story but just needed a few compositional changes.
1) Your secondary element, or the ship in front of the chase happens to be on a gnarly tangent with the background almost every surface intersects in a radial pattern on the ship. The problem here is that your eye totally gets caught here and with the values of the background and the ship being in the same range causes the ship to look connected rather than flying through this sci-fi scene.
2) This ships intersection on the canvas edge is super timid, either crop the ship way off, but still in a readable fashion or move it away from the edge. Also, scale this ship to force the feeling of tension and push your depth from the other ship.
3) At this intersection you have two different planes intersecting but no lighting change to define the form. Even just the subtle little touch on the quick paintover helps these type of issues. Obviously you would want to take it further!
Overall, i would just add a bit more atomosphere to your images to create a more emotional element. In the paintover i may have taken out some of the detail you had established, but sometimes you need to sacrifice all that cool detail to get the emotion, lighting, or atomosphere established.
My little paintover is literally just taking what you already created and just kind moving things around a bit so you really did all the work and are progressively getting better with each batch so keep up the good work!
Thanks-
Levi

Bongo Bong.