Driving around in the Blues Brothers car (or even the D&D dogmobile) might well be fun, but is it really appropriate for a racing sim, especially when there is now an arcade outlet (Horizon) tailor-made for the less serious stuff?
I'm not necessarily advocating eliminating this kind of stuff altogether, just keep it in an appropriate location.
Remember the (correct, IMO) outcry when Soccer and Bowling were announced for FM4? Does anyone really think these added legitimacy to the package, added weight to it's claims as a credible racing simulator? Remember too that these things draw significantly on developer resources. I've always wondered what was cut (Rio? Blue Mountain?) to make room for them.
I've said it before, but if it's Dan's vision to make Forza a car game that is everything to everybody, then there's real danger it'll end up being nothing to nobody.
Maybe a little more focus is required, who knows.