Dry powder ferts! Its difficult to find the stuff at competative prices. I only know of one or two online shops that sell them. Other liquid ferts and traces too.
Also, decent lighting. Not just jo-shmo "tropical" tubes, decent range of good lighting to choose from. This includes compact fluorescents and starters
CO2 gear. Diffusers, bottles, even bottle refilling/trade (like a gas supplier). Also anything else CO2 related.
Test kits relevant to planted tanks - as well as your standard kits, kits for testing that your ferts levels are doing good
Finally, of course, good quality plants and fish! I'm not too worried about oddball fish but would be nice if this shop had a good selection and varied from the usual plants and fish. It would be great if the shop had good plant growing facilities to care for plant stock before selling it on.
I think that covers everything

Appart from stuff to buy, this fictional shop would have great staff that knew their stuff, with a wide variety of experiences and able to advise on different approaches (not just set on one principal and preaching that). And obviously great knowledge of fish of course
Did I miss anything?
