Any update on this in your backlog or ETA?
Got it confirmed as in your backlog for aprox 1 year ago (Custom InputTypes / Schema Layouts)
And its a great feature both for developers and editors to make better UI
For some inspiration on implementation you could look at Sanity CMS doc.
Thanks for the ping Edward, we have not forgotten about custom input types. Looked at this sanity solution this weekend. Also, we will be making improvements to input-sets to make them more user friendly (collapsed by default with intuitive title etc + expand for editing inline)