Completely agree - both about flying birds and fully marine animals requiring aquariums.
Small exhibits (like we have had for nearly 3 years)
birds such as robins, sparrows, jays, finches and the like. You know, all the more common smaller type of birds. I'm totally fine
Fish such as smaller types of fish, jellyfish, crabs and what not. It's harder to explain "small fish" since it's all subject to what someone considers small, but let's say for reference, fish smaller than an adult human head.
Walk through exhibits or larger exhibits the size of the walk through exhibits - other freely flying birds and fish/marine animals that may be around half the size of an average adult human. Obviously the aquariums couldn't be walk through which is why I'd prefer something around the size of the walk through exhibits for those. Birds wise, this is where your toucans, birds of prey, hornbills, parrots/macaws would land. Fish such as koi, and other marine animals around that size. I guess that could be extended to cover marine animals up to the size of an adult human, but i'd be feeling like it would be too small for some of those.
Habitats/Large Aquariums. - for land habitats exactly as it is now. For large aquariums, we would also have the freedom to make them as large as we'd like. Habitat birds would be the birds that spend much of their time on the ground or in the water/water surface. The cranes, storks, swans, pelicans, etc. Frontier could get by on not having these fly, I think, though of course we'd all prefer that, if it's missing from these birds, that's fine with me personally. Think of the Red Crowned Crane for reference. for marine animals, anything over the size of an adult human.