Edit: I don't know what happened to the option to mark the issue type as a feature request.. so it's just marked as a bug sorry

Something I've wished for since MC~1.13 is custom items (NBT/lore etc) with namespaces from datapacks to show in the tab complete suggestions for commands such as /give

For example:
/give @p minecraft:shulke
will show valid tab completion suggestions, but
/give @p mobheads:shulke
will not.

Screenshots below for clearer image of what I mean