If it is going to be anything else, a stripped down version of the current MeeGo-Harmattan would make the most sense. It is relatively well tested, with a good set of default applications. Pushing MeeGo as a featurephone os is probably the most cost effective way to bridge the gap between WP8 and the current S40 line, at least until they can push WP8 further down.
Other alternatives could be using Android, Firefox OS or WP7. But all those also have disadvantages. Android is "the enemy", Firefox OS has performance problems, and WP7 would create confusion with the WP8 line.
They need to get a competitive 100 euro smart-featurephone in the market, with hardware roughly similar to a Lumia 610 (maybe a slightly smaller screen). MeeGo Harmattan could work reasonably well on those specs.

I very much doubt Nokia will be developing a totally new OS, simply too expensive.

EDIT: or it could be based on the OS developed by smarterphone