When it comes to designing a character there are always key things to keep in mind that can help guide a design in it's early stages. I recently went back and looked at the main characters from The Good, The Bad and The Ugly to see if there were some design/character traits that I can take note of to use as influence/inspiration for my own designs.
The first thing I noticed is that the characters are coloured coded via shades of brown, with "Blondie" being the lightest toned to "Angel Eyes" being the darkest mixed in with a few blacks (notably his hat). All the characters have similar roles but with varying details Blondie is a bounty hunter, Tuco is a bandit and Angels eyes is a mercenary, all use guns and can be hired to kill people but it's their stance one morals that separate them.
In terms of redesigning these characters for a sci-fi scenario I'm thinking to change the base qualities, adding elements such as alien designs or species. Already I've decided that I want the narrative to take place in an exotic jungle setting as I feel this is a huge contrast to the original dusty western setting of the original and can really help me develop my designs so that they suit an environment as such.
It's the smaller details such as these that should be kept in mind when comes to designing as it's stuff like this that can really give a good start to a design as setting up some primary boundaries can take away the initial intimidation of a task.