According to
Autoblog there won't be a convertible challenger.
Apparently the platform would require too much additional hardware to handle the extra flex from cutting the roof off, and the added weight would push the car too far into the Sumo class. And that would be with just a simple manual soft-top, not one of them there new-fangled retractable hardtops that are the top du jour for convertible buyers.
There's also the issue of cost and the fact that Dodge's market analysis seems to indicate there isn't a huge market for them.
I figured that cutting the top off would reduce weight. The more you know...