So you want an Amiga 1200 desktop capable graphics card. The easiest way in my humble point of view, is getting someone who has already done it, and modify an existing device.
A good example of who and how:
Elbox has manufactured and keeps selling the Mediator 1200 TX product. It is a pci busboard that supports upto six pci cards, it supports Radeon 92xx pc cards. What if they designed a one slot pci busboard with passthru, in an angled way so that the for example, a radeon pci gfx card could rest inside the Amiga 1200 desktop, parallel to the motherboard?
From a technical point of view, they just need to cut off the extra pci slots, and redesign the board layout to fit inside the A1200 dektop with some kind of angled adapter. Drivers have already been developed for Mediators that support Picasso96, and they wont require any modification.
I believe it could be a big selling product (for just under $200),and require little effort from Elbox to produce. But unfortunately, as a company they have a track record of never listening to suggestions.