I don't have sufficient knowledge of the codebase to write up a full blown proposal, but since GSoC is sponsored by google and for KDE, wouldn't this be a perfect project for it? I seen where akondi devs have talked about GCal support, but I haven't seen anything come of it. Full co-operation between kontact and Google apps would be a dream come true -- When I'm forced to use a non-linux box, I could just open up google and everything I need would be there -- from email to calendars to rss feeds. If only GCal supported "Todo"'s.... I suppose the only problem here is if this project would be doable in the constraints of GSoC.
Shouldn't this SoC be based on porting the composer to be based off of WebKit/KHTML with contentEdtiable? It is the only way KMail is ever going to properly support rich email creation. The current composer is about 5 years behind the times in capabilities because it is based off of QRichTextEdit.