I would sign it, but I don't know if it would have any effect at all. As nontroppo clarifies the API is closed tight, so that means the Opera developers would not consider the addition of an extension system ever. To tell you the truth, I don't think an public API would open the floodgates to migration from Firefox (as people always is saying an extension system would bring the gap between the two browsers), maybe for hardcore users, but it would not be that noticeable. Opera wasn't at the place nor at the hour, so they missed the train.
But enough ranting about unrelated things. I'd like the Opera guys to add real spellchecking for Opera, like Firefox, without relying in Aspell. I think it would be a problem to use OSS-licensed dictionaries, but maybe there's a workaround, like commercial and affordable options, or BSD-licensed alternatives :). I'd also like the content blocker to be more powerful and to work better, Kestrel seems to improve it but I don't know at what level. And something like Launchy (the extension), would be great, again Kestrel has native support for adding at least other browsers, but I'd like to add other programs and sets of defined extensions to launch those programs once I instructed it to do so from an page element using the context menu. And IE QuickTabs, I want that!
(Now someone will come and say: With Firefox and extensions you could have that, etc., etc. ;D)