Personally I think, in the UK (and wider European) market at least, it would dilute the product and the message. A REX is still an ICE engine at the end of the day, classed as a hybrid, and would instantly remove many benefits of the MX 30 being a BEV for example London CC exemption etc.
I've noticed that UK reviews of the MX 30, when inevitably talking about range, tend to fixate on the polar opposite extremes of either "city" driving or that of some mythical isolated countryside. I, (and surely many millions of folks like me) however, live in a sort of country/suburban sprawl, living on the outskirts of a small town with other similar small towns and villages all within a sort of 30 mile radius, with various shops, family, sports clubs, schools, interests etc dotted here there and around. I charge at home, and 100 miles is about my weekly range anyway, so the current MX 30 is a perfect fit. And I do need the additional size offered by the Mazda. Again, the range of the car means that reviews automatically compare it to the Mini Electric and the Honda E, but neither of the those vehicles offer similar abilities in lugging stuff about. I'm not talking about the, again archetypal "trip to the tip", I'm just talking about the weekly shopping.