Going beyond my glorified idealism that I like voicebanks to have, at least, as many phonemes as it possibly can there's been one idea that i've always had at the back of my mind now.
As you most likely already know we've caught ourselves in the generation of new languages, and not just the addition of 3 new ones but the fact that old and new Vocaloids are being produced to be bilungual, having English or Japanese as their additional bank. We've also heard the concept of Nana receiving more, being the French bank (If ALYS did well but, yeah) and a Chinese one. Sure enough some of these new lingual banks don't tend to be of the greatest quality while others can be well done, but my point more so being, how nice would it be if we have one Vocaloid which had a voicebank for each of these languages?
It'd be fairly obvious, just by the sounds of it, that it most likely won't be a complete success given that one would either need to find a singer who so happens to be fluent in all of them (not impossible, but ayyye) or Saki Fujita & Crypton's will power to still make money out of Miku even if they need to poop on what ever language they want to make Saki sing in. But Crypton flaming aside the concept of having a Vocaloid capable of doing that, even to an OK degree would be really interesting to see how well it's taken, especially if it's not restricted to being sold in Japan and whatever eBay seller has managed to get their hands on the products.
Because let's face it, no matter how terrible Miku English turned out to be she was going to sell well anyway because she is Princess Miku of the Vocataku kingdom and that was enough to ensure that she was going to sell. But even at that Meiko and Kaito despite not being marketted outside Japan have been used in the West and Gumi English as well is doing enough to sell (Although arguebly not enough to sadly ensure Lily English and Gackpo English would be a thing afterwards, especially the first as i'd buy that so fast). Essentially what i'm getting at is that familiar faces seem to do well in sales, I mean a Vocaloid you like that sings in your language? Not many would not want that unless you are level 9 weeaboo but anyway.
That being said i'm not so much trying to say that I want Miku Chinese, Korean and Spanish, god no get that thought out my head, or that I specifically want an old Vocaloid to try all languages, a new one would satsify me just as much, provided they have a good voice and an accent that won't kill the majority of languages, at the very least.