For now I would leave the current event kind number specs as it is. It's more flexible that using NIPs for every possible content-type. But I can imagine there will be NIPs around specific content-types which will extend NIP-01 in the future.
Now all the content-type related stuff can be handled by all Nostr clients and map them to the related event kinds. This could be a very interesting mapping to work out (a visual presentation of kind numbers with content-types).