Could Plato have been suggesting the following theory of art in his Ion? What follows is a possible theory of art inspired by Plato, although not necessarily his own.
For something to be art (in, for example, the Kantian sense of "fine art" which is to say, art of genius) it must be:
2. Contain something god-like as the source of inspiration. [The source of inspiration might not be an actual god but rather some person or thing, for example other art, that takes one out of oneself, that causes ecstasy. This thing may be god-like not only in this but in that it has created a world.]
3. The artist must be taken out of himself, must create in ecstasy.
4. The artist enters into a fictional world (as Ion, a rhapsode, enters into the world of Homer) and, for example, feels emotions appropriate to that world. [This is part of what is meant by being out of one's senses.]
5. The artist, in entering into another world, sees our world (or aspects of it) in a transformed way: i.e. he/she takes elements from our world and gives them heightened significance (for example, the poet sees water as milk and honey). In this way or sense the artist him or herself is "holy," i.e. god-like.
6. The artist breaks down the gap between human existence and the natural world in some way. For example in seeing the creek as milk and honey the artist humanizes it, i.e. makes it more intimate.
7. The artist recognizes the limitations of his/her self knowledge: i.e. achieves a kind of Socratic wisdom. This would involve recognition of those realms in which he or she does not have expertise, for example being a charioteer. [This condition is not stated or even implied by Socrates. Socrates, as a character makes a very strict distinction between knowledge based art and the arts of inspiration. But Plato as the writer of this drama may be suggesting this in the end.]
8. The artist does have a field of expertise. For example Ion is able to imitate characters in Homer and knows how to influence audiences just as a doctor is able to influence a patient. [Socrates probably would not have subscribed to this. But it makes sense. Surely Plato was not unaware of this possibility, much as he disapproved of the actual influence of artists.]
for more on Ion see here