Mojang has put up a "do not suggest" list of things that will forever be automatically rejected. You can find it on the feedback site, it should be pinned somewhere close to the top. If it's on that list or seems rather close to something that is, it will never happen. If it's not on the list and doesn't seem closely related to something that is, it may still not happen but Mojang won't necessarily add it at all or any time soon. Reasons for not adding include lower priority, doesn't have the code support to handle the feature yet, or "we just have so much on our plate right now that we just don't have any plans for it".