davidlallen
Deity
What effect does the oasis have? I would suggest adding oases to desert waste tiles (for +1 water) rather than grassland tiles; to grassland tiles, I would add scrub forest (for +1 hammers), and maybe this can only happen in sinks.
Plains, grass and oasis are all copied from vanilla. Desert waste is coast, and no normal improvements etc can be placed there. I agree some kind of forest would be an interesting idea, but I am not sure forest would start in sink. I think it would start on the lee side of mesas. I don't want to use vanilla forest for art, so I'd prefer to keep this for a future enhancement.