So we don't have any spy shots to go on yet but from what I'm reading the designers from both Fiat and Mazda have gone to great lengths to avoid ending up with badge-engineered twins like the Scion FR-S / Toyota GT86 and the Subaru BRZ.
It will not adopt a full-on retro design in relation to the classic 124 but will take some styling from it. It is understood that 90% of the body will be different.
Inside, the MX-5 and the 124 will share a few minor bits and pieces such as stalks, buttons and switches. Major components like the dashboard, the seats and the steering wheel will be designed in-house by Fiat.
The infotainment system will also be Fiat's and is likely to match that found in the new Fiat 500x.