I designed my shield specifically for controlling the turbo.
Putting the canbus on the arduino literally ties up 4 digital i/o's from the get go. Then most people are going to want to at least braking or cruise, etc. So instead of putting all my nuts in a basket I made it what it is, just to control the turbo. If people want more you can put this right on a mega, design another shield (anyone could) and have another side of a whole lot of other stuff. But be forewarned multiplexing is okay, but isn't as great as having dedicated ADCs.
I figure if people want gauges, they would install them or already have them, especially as this is pretty much a standalone controller for the turbo. So anyone that doesn't want to code their own stuff, can literally take my shield, move a couple jumpers and voila, tune the turbo to their vehicle pretty simply.