Well, the exxe IS a "ground breaking" design in many ways.
What made the exxe special is packaging, chassis rigidity, chassis dynamics (suspension), crash strength all in a mass produced car at modest cost.
Previous to the exxe, it was widely believed that the mid-engine chassis layout is not suitable for daily transportation due to space utilization, difficult to service due to power train location and numerous other perceived problems with any mid-engine chassis at the time.
The folks at Bertone including Gandini changes all that. To this day, compare the exxe to any mid-engine chassis for space utilization, ability to carry stuff, passenger-driver ingress-egress, driver's forward and corner visibility, serviceability (some will dis-agree.. try wrenching on a Ferrari or similar), structural strength in a crash and overall chassis rigidity.
These are many of the first introduced to the world as what a small mid-engine chassis could be. It answered and solved the majority of these inherent problems with the mid-engine layout very nicely. Few mid-engine chassis to this day can match what the exxe delivers in a small package.
Porsche 914 appeared slightly before the exxe, but it has a very different mid-engine layout being north-south, vs the east-west in the exxe. After the exxe, most everything else appeared, MR2, Fiero, Elise-Exige-Evora, MGF, Alfa Rome 4C and .....
Bernice
That's tasty.
I don't think of the X1/9 being so ground breaking design wise so much as it was pulling it off in an affordable package