According to wikipedia, by a theorem of Henderson '69, infinite-dimensional Frechet Manifolds embed as open subspaces of Hilbert Space. They need to be seperable & metric. They are generalisations of Banach Manifolds, so they too have the same property.
Michor & Krigel, say 'this does not make them [Banach Manifolds] interesting [enough]' in The convenient setting of Global Analysis,
What are the other directions to take, when looking for interesting infinite-dimensional manifolds, (besides the one Michor/Kriegel outline in their book). And has a canonical choice begun to establish itself yet?