While I would be extremely happy if Toyota and Subaru agreed on mounting a 1.6l Turbocharged direct fuel injection boxer engine in the Toyobaru I really think they could go even deeper.
After reading what BMW says about 'optimized cylinder capacities of 500 cc'
in their new 1.5l three-cylinder engine:
The basic engine is made up of optimized cylinder modules with capacities of 500 cc each, developed to the pinnacle of thermodynamic efficiency for use in a passenger vehicle.
and knowing that a three-cylinder boxer engine is a contradiction in terms
I looked at the new 1.0L, 3-cylinder Ecoboost Ford engine uprated to 205 hp and realized that in order to retain the 'optimized cylinder capacities of 500 cc'
that BMW talks about, the Toyobaru with a 1.0-liter, 2-cylinder turbocharged boxer engine rated at 205 hp
would bring back faithfully the successful (0.8-liter horizontally opposed two-cylinder boxer engined) Toyota Sports 800 lightweight concept of yore
This is already implicitly suggested by Toyota themselves
Also, they could retain the same 86mm x 86mm (bore x stroke) dimensions and consequently the respective logo