Intercooler upgrades, maybe you can make a deal with turbo xs or those knock off companies SSAC and CCS (both of whom do have a good product for cheap) or carry larger TMICS. APS only does orders through them direct now pretty much.
Carry Tatrix cables in stock, that be nice, or the LC-1/tatrix compatible Wideband sensors and gauges. Exedy's better clutches might be nice, so would stocking up on some of Grainger's GM Boost Solenoids (cheap, and exactly the same as Perrins). The local Grainger doesn't carry them (I checked already), although it was some time ago that I checked.
Turbo's would be nice, but I can't imagine how many would move, but then again its one of those things taht if you have it on the shelf, it tends to get people wanting them more than waiting on an order.
More suspension options than Cobb would be nice. Carrying things like the polyurethane bushing sets might be nice to add on to people buying other pieces.