I know the DuckDuckGo discussion if off-topic in this thread, but I'd also really like it as a default search engine. Apple added it to Safari as a default to promote privacy. So unless Opera is sponsored by Google I don't see why it can't be added.
The goal of limiting the list is to counter malicious search engines, so if a search engine has proven itself a reliable and trustworthy, why not add it to the list?