First of all let me start this by saying I've never built a trail, so what I say below may or may not be relevant to starting a trail system.
Looking at the google maps (maps.google.com) of these Metroparks, I think the best opportunity may be the Southern section (south of Willow road) of the Willow Metropark.
The Lower Huron Metropark and the Northern section of the Willow Metropark as much more populated (if I remember correctly) and the river winds and bends in a way that would make it difficult to create trails I would think (many bridges would be required, or you would have to design the trail to either stick to one side of the river or the other).
Now if you find the Willow Metropark on google maps, and then click the "more" button on the top right section of the map, and put a check mark in the "terrain" box - it will show you the topography of the land. There is a 600' ridgeline that runs along the Huron River, which may be suitable to start a trail.
I haven't been out there to see the terrain in person - so I can't provide much insight right now as to whether it would be a good start or not. But I do know that it looks better, at least from the satellite view of google maps, than the other Metroparks in the area, because it is more forested and has fields instead of well manicured grass and a lot of paved trails (which the other 2 have).
Here is a Microsoft Paint cut-out from google maps of the "terrain":
If we were to follow that ridgeline for the most part (obviously we'd add turns and try to use the lay of the land to choose the path so we had more "flow") - could we create something here?
Those are just my thoughts as of right now.... anyone care to comment, provide advice, or give tips?