For this to happen, both Opera and Ecosia need to want it, and it has to be profitable for both.
So did you already contacted Ecosia? What they say about it? Di dthey already try an agreement with Opera?
Google doesn't separate search results that much anymore - like they used to do for different language areas. Sure China still get's the treatment. If Opera continues to be the only browser with forced default engine ~ step 1 would add option to choose a country code (ccTLD) from a short list including .com!