Let's audit:

• Missing block transforms.
• Existing transforms that don't make sense or behave strangely.

Existing

• Multiple => Headings does not make sense?
• Multiple => Columns: does it make sense after a certain number? Which number?
• Does SINGLE paragraph => columns make sense?
• Should there be a quote transform for blocks nested in a quote?
• There is no way to unquote, see also Restore Unquote button #51934. Similarly there's no way to ungroup from the transforms menu.
• Group transforms: we need consistency:
  • quote => group: transforms (see also Grouping a single Quote block transforms it (instead of grouping it) #58042)
  • cover => group: groups
  • group => cover: leaves group
  • What is the right behaviour? Transform? Or group?
  • Maybe we need two "transform" categories: transform and wrap.
• Does paragraph <> code make sense? Also maybe remove Preformatted?
• Should we remove the multi <> pull quote transform (see Having two types of quote is confusing #11610)
• Transforming Image & Text => Image loses text.
• Image & Text => video does not make sense?
• Heading => list does not make sense?
• List => heading does not make sense either.

Missing

• Details block: Add transform from paragraph #55602
• Verse <> Preformatted
• There's no transformations if you select list items (without the parent list block)

Cc @richtabor maybe you have a good sense of what else is missing.