@ExDubai
Did you see my last bullet point above? The lack of commonality is an obvious question in any analysis of a possible partnership.
BUT, the fact is, within the Airbus Group they obviously already have numerous heterogeneous aircraft with no commonality between them.
And more to the point, Airbus already have several joint ventures. Like the 50%-50% JV with Alenia to make the ATR 42/72 mentioned earlier in the thread, and also their 30% stake in DAHER-SOCATA for the TBM 700.
So similarly they can enter say a 51%-49% JV with Bombardier for the CSeries.
Bombardier probably need $2 to $3 billion plus 300-400 firm orders to get the CSeries in sound footing. So it's not a matter of just getting money from the Canadian/Quebec government or from China. As @striker26 points out, they need sales, and for that they need a strategic partner.
IF (and it's a big if) Bombardier can survive independently and take the CSeries to a break-even-point, then there is no doubt that a CS500 will be on their sights. Expanding the product line beyond the CS100 & CS300 is almost the only way they could rationalize the current investment in the program.
If so, Airbus might regret not spending that $2 billion.