Honda has once tried to make a 5 cylinder engine and put it into their cars. The G-series engine family was offered in Hondas and Acuras from the year 1989 to 1998, and they’re probably the most known from the Honda/Acura Vigor and the Acura TL mk1. Two versions of the engine exist, both are I5s, the G20A is a 2.0 litre that produces around 155hp, and the G25A is a 2.5 litre that makes 188hp. Forza tends to feature the more powerful versions, so the G25A would be the go-to option.
While swapping Civics and Integras for something other than a K20 seems simply weird, some people like unusual tunes and that’s what brings in more colour to the scene. Cars like the Acura RSX, the Honda Civic RS or the Honda Prelude Si mk4 would greatly benefit from getting a new engine swap option from their mother manufacturer. A smaller increase in power compares to the already available swaps would allow for more tunes to be made around the cars’ starting PI classes. Maybe it would also allow for a debut of new-to-Forza cars, like the aforementioned Acura Vigor, as the engine sounds would have been recorded.