Assuming that the drain line is 2", you could leave it where it is. If it isn't, then you don't have a choice since current codes require a shower drain to be 2" while a tub can be 1.5".
Centering the drain isn't a requirement, but often, asthetically is the preferred position for it. The drop to the drain from the longest corner distance sets the drop for all edges, so this will make the slope from the shorter corners much steeper, which you may not like.
I like to use Kerdi when building a shower. Check out www.schluter.com for some info on that system and you can watch the videos on how it goes together. They do make a preformed pan with an offset drain location that might work for you. You'll either need a clamping drain for a conventional shower construction, or the special Kerdi drain, if you go that route. You can use Kerdi without their drain by making the pan with deck mud like a conventional shower, but you only need the single layer since you can tile directly to the Kerdi and it goes on that preslope, so there's one less layer to build. When you get to building the shower, after you resolve the drain, check out www.johnbridge.com for help on tiling and build it.