With respect to graphics, we've decided to go 100% top-down like-Alien Breed, Mageslayer or the early GTA games. It means that we don't have to draw out all the frames for the different orientations and I believe it fits the overal vertical shooter theme.
Here are some examples of games with a top-down view:
Alien Breed (1 & 2)
Here's an example by Niklas Jansson of how good top-down can look - now if only he'd work with us on Soulreaper!