I would love to see Ashley return to the Mass Effect franchise, that's something that has been annoying me for some time. Yes I know she's not the most popular character but damn it from my perspective Ashley was actually one of the best female characters in gaming and partly that was due to her flaws. It made her feel more human and gave potential to character development, plus she wasn't tarted up as a sex symbol.
I want my sexy space marine back, damn it!!
Also, I want to see a Halo game made solely about the Elites and the Covenant. We've done the UNSC to death now, how about we go have a look at the other side and do it properly? There is a whole untapped world of content in the Covenant's story that is just begging to be told and gameplay mechanics that could redefine the Halo experience.
That's my two cents anyway.
EDIT: Also I just thought of something else.
I'd like to see female aliens. By which I mean for the alien races in games that clearly look nothing like human beings, I would like to see both genders. I mean look at Halo, in official canon we know that there exist female Sangheili, Kig Yar, Unggoy, Jiralhanae, San Shyum, Yan'me and (possibly) Mglekogolo but where the hell are they? We only ever see the males of the species even though they are supposed to represent the whole race we never even see so much as a female Sangheili working a job as a waitress in the background on High Charity.
Now look at Mass Effect, am I the only one that thinks it's odd that no matter what planet or space station we go on we have yet to see a female Turian? Where the hell are they? Salarians, Batarians, Krogans, Elcor, Volus, Drell and so many others... all male without females.
True we've seen both male and female Quarians but they are humanoid so it hardly counts and whil Hanar, Geth and Asari are all off the hook because they are probably mono gendered it still begs the question as to why I cannot for the life of me, see a single female alien that isn't humanoid in all of the Mass Effect universe.