Concept

Build upon standard E3D specifications
Add Scalable Heater Block made of multiples of existing blocks for low cost
Add a rubber-optimized 3mm extruder. Make extruder flexible to larger filament size
Add larger filaments, such as 4-5 mm Shilament from the open source Filament Maker.

Experimental

Adding heater blocks as in Scalable Heater Block requires only that we figure out how to face a hollow brass rod to connect a second heater block if using 2 hollow rods
If using one rod, it's possible to align both heater blocks as needed by usng the heat break with adjustable endstop (nut on break, or epoxied. This is feasible only in a redesigned extruder.
To use a stock extruder, we can use a random angle, but cooling fans would have to be redesigned for a random angle. Tbis is the easiest route because we have control over cooling fan design. This route would require no.other change than adding safety stops on X and Y axes to prevent breakage of heater block in fault conditions.

Comment: the following design is.not good for rubber, needs shorter distance between drive gear and hot end.