Are you going for game concept art or the actual development?

hmm, if you asked me, i think you should experiment more on your environmental art, you do a great job at them so far, honing it more will do you good, especially for portfolio. Landscape artist is so rare.
(Heck i need to do more practice on landscape as well -A-;; )

I think environments is a good recommendation. And if you think you might be working in a game with items and how characters use items...maybe show some models of characters using/holding items from different angles.

Post apocalypse is always a fun theme! Mutated creatures and people could be a great theme to work with.

I would recommend environments; nature/landscapes, underwater, cityscapes (post-apocalyptic, futuristic, current). I do have a few mountain stock photos that I took myself to use for refs for a painting a while back, I'd be happy to find them and share with you if you want =)

it depends on the company you're shooting for. when you know what company you want to work for, then you need to figure out if you want to do characters/environments. props and assets also fall into the environment category; there are usually just the two: character, or environments.

then, you need to know that company's art style. in the game industry, every existing company has their own specific style, and will be expecting you to adjust to their way of design. if you know your company's art style, and can replicate it effectively, then you can move onto where you want your emphasis to be. if you want to do characters, think of characters that would fit into one or more of their games (if they have multiple games) and do conceptual designs. show your thinking process, show your ideations and your ability to think about a successful design and provide a final concept render of a character. this is the same for environments; treat your environment in the same way.

practice your skills like the artists who already have the jobs there do. study their work. figure out the way they do things, so when you apply somewhere, you know what they'll be expecting of you.

a word of advice: if you're going into character, you need to show that you know exactly what you're doing. getting a job in the character concept/design department of any studio now is almost nigh impossible for someone new to the industry, but it isn't impossible. you just need to be damn stellar at what you do. industries will expect you to already know about research, concept, traditional mediums, building up a broad visual library, knowing your audience, and tons more. one of the biggest things that companies look for is being able to successfully create appealing and memorable characters, and that their stories can be deciphered just from the design alone without explanation. this is a very difficult thing to do, but this is the heart of concept and design.

if you're going into environments, the same thing applies. not only that, but the environment needs to be treated as a character itself. this also applies for props and assets. questions to ask yourself are: what is the story? how can i portray this story? what did this character/environment encounter, and how can i portray that idea into the design? etc. etc.. the list can go on.

another word of advice: learn your 3D, and learn it well. many, if not all of the companies in the game industry now are starting to fuse both concept and 3D together into one profession. conceptual 2D-only jobs are becoming extremely scarce, and are being reserved for veteran game artists nowadays. to get the 1-up in this case and get yourself into a better hiring position, on top of your 2D you're going to want to start getting better with your 3D. in the game industry, a concept artist that can draw out AND model their own concepts and take their models to finish shows so much more potential that someone who can only do 2D artwork for a game. the ability to do both 2D and 3D shows your employer that you have both the mind and matter to transfer three-dimensional shapes and anatomical forms into a 2D space, and recreate them into a 3D space. that kind of skill is hard to find and is being heavily sought for right now above those who can only do one or the other.

the biggest skillset, however, that anyone could ever have that would put them even FURTHER than their competitors is the ability to design. this is something that takes time and patience to master, and it's the hardest thing to accomplish for any artist. anyone can learn how to draw, or paint, or sculpt... some learn faster or slower than others, but it's something that can be taught. the ability to make a successful design, however, is something that's not so easily transferable from a mentor to a student or peer. design is downright difficult just to teach, but it's something that is built up and strengthened over time. design is the thing that takes 70% of the time when creating something, be it a character, environment, or prop/asset. it's the research and the referencing and the thinking and the processing of what the thing in question is going to look like, why it's going to look that way, the reasons for why it does the way it does, and so forth. the other 30% is just making the art happen.

not to burst your bubble, but asking others what they think you should draw won't help you in this instance if you're making a portfolio for a company in the game industry. this is basically reliant on your own knowledge and your ability to research your target company, as well as your own personal tastes/instincts. basically, trust your gut. if you know your target company, and if you know what they're looking for, do exactly those things in their style so they know you know exactly what they're doing, what they're looking for, and that you can replicate the artwork they produce. that isn't to say that people can't help you along the way, but it may be up to you to get started to know exactly what you need to be doing before you can start getting help along the way.

i know this is a long read, but i like to help those who are getting into the industry i frequent in. i speak from experience, and a lot of people don't know this information and end up becoming lost or tied down with artwork that won't assist them in getting into the industry. i don't want that happening to you, so i'm giving you all the advice i can to get you started! you're a great artist and i think you have a strong hold on design, you just need to know where you want to go and start applying yourself to the company you want to work for!

if you'd like, i can hit you up on Skype with links to very successful game artists and designers so you can start researching their artwork, and i can help you pick through the work and start getting yourself prepped for a portfolio. i'm also available if you need any more advice on this kind of thing and hopefully i can point you in the right directions based on my experiences!

I'd agree with most here and say that your best bet is environment and location.

Example, every Mario game has wicked and crazy locations, they are all connected somehow, but the environment and how things flow from one to another is just awesome.

If you want to play around, do different worlds if you want. Try different scenery you aren't familiar with. A challenge to get better, but also a challenge to explore. Just don't set the bar so high, keep it attainable and work from there, up.