@makekong wow I think you have a good taste in covers, those multicharacter covers you shared are all so fantastic. I must admit that my favorite are the batman black and white and the batman two-face--maybe all the batmans haha. I guess that tells you about my personal taste. The common theme that I am seeing is they all have very clear size variations, conscious silhouettes, and they use the whole space in one way or another. I think this is something you have a lot of room to play with still and it would really strengthen your cover, (all your characters are roughly the same size and tend to be placed in the center of the page). The covers you shared were made with a predominant character and a second-read character(s) that is much smaller. In the case of Dragon Ball Z covers and the Fantastic Four cover they are showing a more dynamic moment--a complete action idea, but they still have a predominant character, and the smaller other characters, all with a very clear silhouette. Another important difference between the ones you shared and the thumbnails you have is how much space the text takes up.
also, be careful not to fall into the contemporary movie poster trap that is just a bunch of people collaged together. They do that to advertise the famous people in the movie (big faces, and lots of small supporting character faces), but the reeally good book covers and comic covers (and movie posters, lets be real) tell you something about the story that grabs you by making an emotional connection with one of the characters.
the only manga I have ever read was sold to me in these two images, one by an artist I followed
and the second was the cover of the first volume that I saw online
I just had to know who these people were and what their story was!
hope it helps! im excited to see what your next steps are! it is hard getting stuff out of your head, but I know you can do it!
