I heard someone at a conference (not a speaker) try to explain this idea. I
don't think there is a problem creating multiple projects under one solution and
making one big manifest.xml file though I haven't done it or seen it. I would
love to hear what you come up with!

You can definitely create solutions that deploy  multiple features.
Stapling and feature  Dependencies are what you are after.

The only problem I have run into with Feature Dependencies is having
Features at different scopes can get a bit of a tangled mess L

blogs.msdn.com/.../feature-stapling-in-ws
s-v3.aspx

msdn.microsoft.com/en-us/library/aa543162.aspx

You could even just create a solution which contains all your features
and then one quasi-stapling feature to activate them as needs be.

www.dotnetmafia.com/.../ho
w-to-programmatically-activate-a-feature.aspx

Yes, This can be done easily using Feature Stapling.