I think that you have too options for a PTO collector to go behind a tractor.
One has a rotating brush and simply brushed everything it goes over up and pushes it into a hopper.
Look something like this
Our local rugby club uses one and during the Autumn covers about 50 acres of thick leaves that fall a week. There quite effective.
The other option is more of a hoover.
Something like this
There website is
here
This might be easier to make. You would just have to have the fan unit and then a massive hoover head to drive over the leaves with.
What are you going to be using it for?