Regarding the issue of "rich multimedia" clients like Viber and WhatsApp, it seems to me that the incumbent operators start to get concerned about these applications taking a big dent in their SMS-Voice revenues.
They are all commited to a new type of applications called RCS-e (Rich Communications Suite - enhanced). Google for it to know more about it.
There are many RCS-e clients from many vendors (including Nokia, Ericsson and others), and I think it would be nice to port them to maemo.
Some of them are open-sourced as specified here before... http://talk.maemo.org/showthread.php?t=70433, altough I think this doubago stuff is a bit outdated.
Anyway I think an RCS client ported from linux to maemo would be nice and expand the lifecycle of the product.

Regards.