Investing in original contents in Malayalam is very difficult since Zee may not able to crack deal with production houses.
Zee guys made their mark in highly localized competition in Tamil & Telugu market so Malayalam won't be a problem. They have deep pockets so cracking deal with production houses won't be a challenge. Even buyout of an existing production house is possible. As it is all 4 south geners are interconnected in terms of technicians, studios, etc.... So time will tell what's in the cards...
But yes zee had even got licence for Zee malayalam in past. There were similar announcements over 2-3 times in the past which did not work out. As latest as 2016 it had expressed interest in inorganic growth in Bhojpuri n Malayalam market. Bhojpuri happened via Big Ganga but Malayalam didn't take off.