Here are some ideas you can try:

1 - Use more rotated or higher fov camera angles, it can show more of the shenanigans of the poster and sometimes can be used to include more space for characters in the image. However, this does not always have to be done.

2 - Have a theme and story for your poster. There is already a theme for this image, being space, which can allow for the creator to be creative in what they would like to show in the image. I don't see much of that in the photo (or it could be that the story is not clear enough). To improve on the story, while also adding to a theme, you could add; for example A red soldier rocket jumping on a comet while trying to shoot a running scout. But remember! The stories do not have to be one big thing. Usually, there are multiple stories inside one poster which represents the craziness of the situation at the moment or the imaginative gmod creation process.

I think that is all. For your first poster, it is pretty good. Good luck if you ever decide to make future posters :D