You'll hear many concept artists refer to themselves as 'problem solvers'. I think it's a key thing to keep in mind when you're thinking about doing this kind of job: can you use visual communication skills to solve design problems? Every studio and every gig has it's own list of demands and different artists are suited better for certain jobs over others, but at the core of it whether you're an environment artist, a character artist, a prop artist, etc., you're job is to come up with design solutions for those things. If you're working on a portfolio, many artists recommend you gear your work towards what you actually want to do. For example, If you want to design spaceships for a living, make sure your portfolio includes cool designs of whole lot of spaceships! Different kinds, different shape languages, different use-cases. Make design problems for yourself, and show that you can solve them. Your portfolio is there to show potential employers what your skillset is!