Without a doubt, on the English side of things, I think Cherry from Studio Killers would be a really cool idea. She'd be basically the English variant of Lily mascot-wise, and her creator consistantly posts art of Cherry on her blog wearing all sorts of crazy creative outfits, so it'd be very interesting to see how the Vocaloid art would look. I feel it would also boost Vocaloid popularity here in the west seeing as Studio Killers is fairly popular, and it would show to some people that it's not all SUGOEEEKAWEEDESOOOO weeaboo stuff. Voice wise, it would no doubt add some much-needed variety to Vocaloid's selection of libraries - no one can argue that Cherry has a very unique voice, bordering on androgynous, and having very unusual pronunciation at times, it'd be something I feel that producers could potentially have a lot of fun with if the voicebank is done right.
Now, for the Japanese side, I'm going to let out my inner weeaboo for a second, because the reason I want this one is totally stupid, but, I'm going to go with Sumire Morohoshi. Now here's the thing, I know very little about the world of Japanese VA's and singers, so the reason I picked this one was because of a character voice she does. Now gamers may be aware that the game "Tokyo Mirage Sessions #FE" recently came out in the west, which is basically a Persona game with a theme revolving around the entertainment industry, and the there are some characters with the likeness of Fire Emblem characters ( who basically replace Personas and Shadows from the actual Persona games ). It's great, and I, among many others, are having a great time with it - however, there was one character who caught my eye a lot throughout playing it. Namely, Tiki - which if you don't know, in the original Fire Emblem games, she's a Manakete, a humanoid being with the ability to transform into a dragon-ish sort of form with the help of a Dragonstone. In this game however, she takes the place of the Velvet Room from Persona games, and fuses materials together to create new skills and weapons for the characters. Now character-wise, this is where it gets interesting. In the game, she is also the mascot for the software "UTA-LOID TiKi", if that doesn't already sound familiar, it gets better, throughout the game you see advertisements for the UTA-LOID software, and at some points, you can even see that in-universe, she even has a physical boxart, there's even one quest where you have to rescue a music producer who uses Tiki's software, who's producer name is "Tikiismywaifu" or "Tiki=Waifu" for short. Now perhaps needless to say at this point, in the game Sumire Morohoshi is her VA, and Tiki's songs in the game, also sung by Morohoshi, sound pretty good, if not a bit generic. As I was playing the game, I couldn't stop thinking about how if they made Tiki's mascot into an actual Vocaloid to promote the game, seeing as there is no way in hell it will suceed in the West, and it certainly didn't in Japan I don't think, seeing as there was very little promotion, and not much localization was done other than changing the dialogue to English and censoring stuff, the latter of which became a major source of controversy, and deturned off a bunch of potential buyers for a game that already wasn't going to sell well, that it would not only boost sales of the game slightly, but would help them maybe make a little side money while we get some cool content. With Atlus being a gaming company and Nintendo owning the character Tiki however, it'd probably never happen. Overall though, it would be amazing if something like that could happen, that being adapting Tiki's voice into an actual Vocaloid.
Alright, now I'm just going to bury my inner weeaboo waaaaaaaaaayyyyyyyy back down and never let it come out again ;D