Yo. Software engineer here, I do this kind of thing for a living. If you're serious about an open-source version of UTAU, I'd recommend at minimum putting together a team with the following people:
1. A leader
2. A user interface/visual designer
3. Two or more devs
Unlike a plugin or resampler, UTAU is a public-facing program with a full user interface, making it much more tricky for a single dev to implement. The hardest part of making this kind of software is the non-programming part. Some challenges I could see popping up:
- Motivation. This is why a charismatic leader is a must, preferably one who has led projects of this scale to completion before. They would have to keep the project from dying and the team from losing interest.
- Publicity. If people don't hear about the product, don't use the product, don't care about the product, it will die. There's a reason software companies have marketing departments.
- Product design. If there's not a clear and documented vision for the software, devs won't be able to build it. What will it look like? What framework should it use? What languages should be used to write it? Should it be a web app or desktop app? How will it support Mac and Linux users if it is a desktop app? Should it include any plugins as built-in features, and if so which plugins? How will it support different recording styles and languages with Unicode characters such as Japanese of Korean? If a plugin/resampler has code specifically to work around failings in the original UTAU, should the new software duplicate those failings so the same plugins can work? Should oto.ini file size limits be removed to help along more advanced UTAU like Beta? How should plugins work for non-Windows users? Etc, etc. The problem space here is enormous, and much of it has nothing to do with coding. Most professional teams have a dedicated Project Manager (PM) who does nothing but work out these annoying issues all day long.
TL;DR quality software on this scale needs a team with a good leader. Visual designer is a must as well. If you guys are serious about making this happen, let me know and I'd be happy to help write the thing. I love UTAU and this community, even if I am mostly a lurker.