Like most of the ideas in here. Would like to add a set of smoked/normal rear lights that are all the same damn color. Every pair (10+) of 89+ lights I've had have had an off color center section and it drives me insane! Would like to see some smoked lights that match the smoked clear fronts/side markers already available in addition to a set of normal matching rears.
On that note (and encompassing the pop up delete idea above), some modded running lights with HIDs
No goofy looking mods that replace/fit into the stock pop up housings, but into the running light location. Something I've been thinking about doing for years and never got around to.
Would also like to see T4 manifolds. Even if you literally just took eBay chinafolds and did the necessary mods and reinforcements to make them not suck. As much as I'm sure everyone would like to run RPS/F1 style manifolds, paying 1600 for a part when you can spend $200 for the same result is a pretty tough sell for many people.
As mentioned above oil cooler kits that are bolt on affairs would be nice. On a similar note a 1.5JZ kit would probably be a slam dunk as well. Timing belt, idler, sandwich plate with two ports for oil gauge/feed line and a drain flange with included lines and an extender piece for the water neck (seeing as most people who go this route use NA blocks AND most of this would be applicable to NA-T guys).
Preconfigured bolt on IC and pipe setups. For example, a single turbo JZ car needs two IC pipes. A 7M/ NA-T 2J without an FFIM needs three. Not everyone has access to a welder and this is usually a PITA to get perfect anyway. Weld a small bead around the ends to help keep the couplers from blowing, add quality couplers and clamps along with it. Include a choice of BOV flange and bovs. You could sell these with the manifolds and oil lines mentioned above and either supply the turbos so its a complete kit or allow people to source their own.
Fuel system upgrade kits. Billet pump holder assembly (single or dual) with -8 or -10 fittings already installed, premade feed line/ lines with fittings and filters up to the rails. Wiring upgrades. Bolt on ready to go packages for 500/750/1000, E85 compatible, etc. Pump holder assembly could be done in two pieces, have the cap made and a pump holder rod that screws into it. Would make them massively cheaper and easier to make.
Transmission adapters. 7M, JZ and UZ to GM and VQ(350Z six speed in particular) with any and all required hardware.
A whale tail/rear lip spoiler as mentioned above. Something a little less aggressive on the sides, three piece that bolts into the stock location + some type of gasket to keep it from leaking.
Many of these options would benefit more than just MK3s as well which is never a bad thing.