I have looked into this a lot myself
A few things i decided on for my own part
1 It needs to be based on MIDIbox (
www.ucapps.de) the knowledge base and kits available are just too cost effective and powerful (prototyping and experimenting becomes very easy)
2 Modular in nature with the correct split of modules, for instance never put transport controls on the same module as the faders (Who wants to build two fader modules each with 8 faders and have two sets of transports)
3 Cost is of vast importance, while 1 or 2 people may well want to go full on and have motor faders and so on, the vast portion of Reapers user base would mainly want a cheap alternative (No matter how well it is intergrated)
Because of the costing side of things the important part to work out is the fader board module because you need a list of options as related to cost, the options i came up with where these
Budget option (est cost = $100)
Rotary encoders with shift button for precise adjust, Very low cost option can be built for little outlay
Mid Range option (est cost = $200)
Hotpots over LED (See Stribe project) high precision touch sensitivity with the added advantage that it also works as VU
Expensive option (est cost = $350)
Touch sensitive motor faders, only real issue with this is one of cost, with a massive group buy possibly getting ALPs motor faders down to about $18-25 each (This would need to be a big group purchase)
The estimated costs could well be wrong now, it has been a while since i looked into this but i do know that as motor faders are gradually being faded out in industrial use in favour of hot pots and other touch devices that they are growing now in cost on an almost monthly basis
Not to put a damper on the idea (Far from it, I had already decided years ago that the only real way to get good hardware control of Reaper was to do it the Reaper way and build your own) but as you can see from the estimated costs (That are probably very conservative now) it isn't really a cost effective option for the budget user considering the budget controllers available. problem being that to bring down the overall cost of a project like this you would need to cover the base common user, which in Reapers case is the budget user.