The blue sky is a great touch, and the sense of movement here is fantastic. I love how you've done Chell's face, making her look a lot more open, and, well, optimistic than she looks in the game. I think you're right to call her the "new Samus" (*prays I'm translating it right*) as well! I was never really into Metroid, but with Portal I can be proud to see a female character as awesome as Chell. Beautiful work capturing the game in a unique style of art. I love the painterly look of it and all the different ways the light from the portals interacts with her clothing.
I like that this renders out from the rest fanart with a happy, bright look to it with the hopes of an open sky above Chell! Wheatley looks a bit stiff compared to the relaxed smoothness around the rest of the drawing. But I ESPECIALLY LOVE the determined look on Chell's face. You got that spot on. Absolutely brilliant.