Hi there. I've played your game for some time and I think it's a nice idea. But it's too calm.
The flying feeling is good. You need to keep your locations high to prevent hitting the floor every time someone's dive. You can make some diffcult tracks with many obstacles, but also keep "free ride" places around.
I'm looking forward to some various animations of the bird. I really feel like flying a bird, but good animations will make it more intense. Also particles and blur have to indicate your trajectory and speed.
I don't know if it's a bug, but not all items can be collected properly. I think it can be due wierd hitboxes, so work on it. And one minor thing - I couldn't steer right after flying straight forward for one or two seconds, but steering left was alright, moreover when I steered left a bit I could steer properly into right direction again.
If you ask about the world - keep it magical. Modern, but magical at the same time. Best example:
About art style - it doesn't matter. What matter is quality of it. I think something dream-like would be fitting, not so realistic, but not cartoonish as well.
You've mentioned co-op features and I have some great answer for you. Make mini quests, adventures. For example, I'm connecting into one game with my friend and we find out he lost one eye or body colour or tail's feathers and we have to find it in this beautiful, magical world.
I'm really convinced to your idea, I just think you can make game better if you follow my advices. I'm really looking forward to see the full version. And forgot one thing - you can add option for importing own music, it could be great in full game.