I think they should allow every one to invest in stores, and finding something else to reward master merchants with. If you are going to spend your time increasing a skill as useless as mercantile, you should be rewarded with something better than an increased maximum pay out on the items you sell.
I would definitely be down for something like this, maybe making the risk of investing higher for players with lower Speechcraft/Mercantile experience. And then once you've actually 'mastered' it, you can take it to a whole new level. Maybe having a building, deciding what wares you want to sell, and hiring an employee/security.