Each studio has a set amount of plans for future Vocaloids. This does not mean it's impossible to become a provider, as some studios have now done contests to choose who becomes a provider.
If you are a native English speaker then chances are you are restricted to vocalisig a English Vocaloid. So far there has been no opportunity to be a provider for English Vocaloids. Before you get your hopes high, be aware that English takes a long time to do. Even if you ever become a provider you've got to sit through many hours of recording sounds in different keys. This will happen no matter what Vocaloid language you do and will take at least 4 hours (without any breaks) to do.
UTAU is a free alternative to Vocaloid and you can make your own UTAU with your voice for free.