Hi @benjamin.martin,
I think this feature would be prioritized. I'm building from scratch a huge website (custom plan) with 50 slices and maybe 20-25 content types, and I have problems in almost any slice to build the correct model because I can't nest groups. I've ben using the undocumented way to set nested groups (just one more level) and it worked great, but even with this trick is not always enough. Sometimes I would need 3 or even 4 levels of grouped content.
Current way to do this is to create more custom types for every card or portion of content, and then use relationships, but even this way it's impossible to handle it a lot of times.
So we are currently changing the visual design and UX to avoid more than 2 levels of hierarchy, in my opinion this the worst scenario for a designer and a developer, where content model is the blocker to build something, not any technical or business reason.
I know I don't have to use that trick for nested groups but it's the only way to at least save the problem in a few places and avoid the client to add new documents, the website will have more than 5000 documents in 12 languages, I can't say to my client: "You have to create 1000 documents and the only reason is there is no nested groups in Prismic".
Now if I start using cloud Type builder, I'm afraid nested groups can disappear, at least with local generation I can control what can I do with model building.
So, after building at least 30 Prismic sites, since a lot of years, long time before SliceMachine even born, I please ask you to take this feature and solve such a common problem for all people using Prismic. In my opinion, if you ask me, this is the only thing I could change of Prismic right now.
About your approach creating specialized fields (I guess you talk about repeatable fields like LinkField or ImageField in the future), these are great to improve the content builder UX, but not enough to model content, maybe I need to repeat number and currency fields for a list of prices in a list of cards, in a list of tabs, specialized fields don't help to build this.
Once again, thank you so much for all your work, Prismic is such a great tool and you're doing an amazing work.