#1 LSD for a reasonable price, $1800 only if its a clutch type, no more than $1500 if it isn't. It shouldn't be too pricey since the production runs on such a part are going to probably be greater than what Wavetrac could produce (though I am still waiting on Wavetrac at the moment for their automatic differential).
#2 make such things as boost pressure, tire pressure (for all four tires individually, hell Dodge's and Chevrolet's have it on their cars that cost far less), oil pressure, air/fuel ratio, voltage, etc available as added menus on the Performance Steering wheel. If none of those, at least boost pressure and oil pressure. If not maybe a gauge cluster where the ashtray is currently located. (no more than $)1000 for the cluster and gauges (no more than $100 if the panel is made out of carbon fiber).
#3 Make non-runflats an option with a kit similar to ContiComfort. Make the M series tires available for non M cars. Non-runflats should be made standard, with runflats as a convenience option.
#4, Performance seats with harness and harness bar. (no more than $1000 for each set, no more than $2000 if fitted with airbags as standard) should probably use same setup and type of seat as (featured in the M3 GTS.
#5 Uprated performance turbos (i.e. bigger turbine wheels or a sequential system, one small turbo for daily driving scenarios and one big one for when you get on it) or software (reflash of the current Performance Power Kit) Cost is up to BMW, no more than $3k but must make significant gains (around 400hp/400 ft.lb. torque)
#6 Make real lightweight wheels, the style 313s aren't that light and cost more than wheels that are a good 6 pounds lighter.
#7 A bolt-on rollcage (4 or 6 point ) would be nice to see (no more than $1500 primed and painted)
#8 A strut bar that actually clears the stock airbox, its not fair the 128i get it when most of the performance minded buyers own cars with the turbo motor. Also a rear strut bar would be nice to see.
#9 An intake kit similar to Dinan,still a closed intake, but with a cold air intake (something that can also route around the BMW Performance Power kit) or a ram air intake sort of like the one on the M3, with hood as well.
#10 Vented hood, with possible closable vents either by user or automatically as with the rain sensors.