There are going to be some changes in the upcoming chapters that will make space on Diagon for new businesses.  However in this universe I'd like to think with all the changes being made to Knocturn that the twins would want to open up a shop there, especially seeing as how Knocturn has always been forbidden to them and that's just the kind of thing they would do as soon as they're of age.  It will all be more clear when you see the changes happening in the next chapter.  A new map will be coming out too.

Coupons are a great idea.  I never even thought of that.  I'll have to think about how Harry would incorporate that into his plans.  I also hadn't thought of advertising.  Maybe Harry could get an ad on wizard radio or put fliers up in the Leaky Cauldron.  There's all that space on the brick wall behind the Leaky Cauldron where fliers could maybe go...