I imagine the costs of putting a bigger engine in the i8 will be very prohibitive given how expensive the car is already. A V6 is probably the best option given the width restriction and having to fit a gearbox in to drive the rear wheels but BMW don't have a V6 that I know of so that would be completely new as well.
The best bet for a fast i8 I believe is full electrification when battery energy density allows decent range and also keeps the weight down enough to retain the handling.
A retro-fit kit in the future would be interesting, expensive and need some clever software but physically I can see it working.
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2014 i8
2002 x5 4.6is
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