Let me say something right now that CAN be considered a jump to a different sub-topic under "what do I think about appends"
Don't make a new design for every append you make.
Like seriously, if you have to make a new design for your UTAUloid's appends, only make one append design. Like Gumi EXTEND, she had one design for it, four different poses.
And even then the design isn't all that important cause creative freedom. How often do you see, I don't know, Miku, in her original outfit, seriously
Upon the note of actually HAVING appends for an UTAUloid, I think the idea is just fine! However, you have to understand somethings.
1. An append is an append. Its not meant to sound totally different from the original voice. Its simply meant to change how the timbre of the voice sounds to fit other genres.
1b. This argument can be argued against, but in all seriousness sounding similar to "other appends" is a good thing. Its not a good thing if its EXACTLY THE SAME, however. Miku's softer appends go like this, DARK: 10% breathy, low tone; Light: 30% breathy, middle tone; Soft 60% breathy, high tone: this is something few people understand, is that the appends also are recorded on different timbres, which is something that is hard to control without practice. If you're recording an append that you think sounds too similar to a previous one, you really need to do some sample-comparison before releasing a new one.
2. Don't think just because your append is complete that your core bank can now sing those genres. If it can, you integrated the suffixes, congrats. Usually, people don't do that.
2b. Don't ask for examples. I'm not going to drop names.
3. Ever get the feeling the append doesn't sound like the original voice at all? Too much variance can actually blend in with the original voice, proving you wrong.
3b. There was a video where someone combined all of Miku's appends + the core some point after Miku Append came out, and it sounded like she was changing her timbre instead of singing separately as each append, like a chorus. I'm pretty sure it used voice-morphing instead mixing together, so that probably effected the result, I'm pretty sure the result would've been totally different if even one sounded too unlike Miku.
...now someone is going to swoop in, quote me, and find some way to make my argument invalid. Its not an argument, its an opinion, and I know some people will jump on twitter going on about how ridiculous my opinion is (because yes, this has happened before, I -stalk- twitter, don't run away and criticize me behind my back), but the problem is -its an opinion- so there is no right or wrong. Thank you kindly in advance.