Will manufacturers be interested in allowing a flat 6 engine

[Jolle]

With no restriction on configuration, would they run an inline-four, inline-three or even twin-cylinder? It should reduce friction losses, right?

Good chance indeed they would run at least a inline 4 or a V4.

Boxer engines in road cars have a lower COG then V or inline engines because there is enough space to do all the piping under the heads because of the sump and road engines have less extreme intake channels.
If you go for a dry sump, the engine needs to be raised from the main plane to have exhausts (or you have to do some weird porting like mentioned above) and the intakes will be in a far less desirable place.
Ever since we understand the importance of as straight as possible intake tubes and the narrow coke bottle shape of the car, a V with the intakes in the center is the only logical conclusion, with added bonus that the V shape is perfect to use the engine as a structural member.

So yes, chances are they considered it, but only very very briefly... around 5 minutes.

If they would switch to twins on the other hand... then there are possibilities (

What would be the odds on considering a transverse engine? Maybe a 3 cyl?

As long as you can have the balance between good piping, COG and packaging, anything is possible. A 6-boxer just doesn't fit those demands.

[Big Tea]

Good chance indeed they would run at least a inline 4 or a V4.

Boxer engines in road cars have a lower COG then V or inline engines because there is enough space to do all the piping under the heads because of the sump and road engines have less extreme intake channels.
If you go for a dry sump, the engine needs to be raised from the main plane to have exhausts (or you have to do some weird porting like mentioned above) and the intakes will be in a far less desirable place.
Ever since we understand the importance of as straight as possible intake tubes and the narrow coke bottle shape of the car, a V with the intakes in the center is the only logical conclusion, with added bonus that the V shape is perfect to use the engine as a structural member.

So yes, chances are they considered it, but only very very briefly... around 5 minutes.

If they would switch to twins on the other hand... then there are possibilities (

What would be the odds on considering a transverse engine? Maybe a 3 cyl?

As long as you can have the balance between good piping, COG and packaging, anything is possible. A 6-boxer just doesn't fit those demands.

A flat, or slight V sideways 3 or 4 would seem to be a good base for re-arranging components down low, well forward, and possibly having radiators above.

Exhaust from cyls pointing toward driver seat, into turbo at the same height as the engine, then down one side. It could even mount the rotors horizontally to take up no more height than the 'engine'.

[Jolle]

What would be the odds on considering a transverse engine? Maybe a 3 cyl?

As long as you can have the balance between good piping, COG and packaging, anything is possible. A 6-boxer just doesn't fit those demands.

A flat, or slight V sideways 3 or 4 would seem to be a good base for re-arranging components down low, well forward, and possibly having radiators above.

Exhaust from cyls pointing toward driver seat, into turbo at the same height as the engine, then down one side. It could even mount the rotors horizontally to take up no more height than the 'engine'.

Possible, but also take into account that "the heavy stuff" needs to be in the center as much as possible to keep your inertia down. Space then occupied by exhaust should be used by for instance the batteries to have the car turn faster. This is one of the few downsides of the spit turbo from Mercedes for instance and possible makes them a bit less "sharp" on circuits with lots of fast direction changes.

[Big Tea]

As long as you can have the balance between good piping, COG and packaging, anything is possible. A 6-boxer just doesn't fit those demands.

A flat, or slight V sideways 3 or 4 would seem to be a good base for re-arranging components down low, well forward, and possibly having radiators above.

Exhaust from cyls pointing toward driver seat, into turbo at the same height as the engine, then down one side. It could even mount the rotors horizontally to take up no more height than the 'engine'.

Possible, but also take into account that "the heavy stuff" needs to be in the center as much as possible to keep your inertia down. Space then occupied by exhaust should be used by for instance the batteries to have the car turn faster. This is one of the few downsides of the spit turbo from Mercedes for instance and possible makes them a bit less "sharp" on circuits with lots of fast direction changes.

Good point, I had not considered batteries.