I have a set of Frogg Toggs which are quite breathable but I'm having the same thought processes you are. In warm weather the heat builds up inside the suit and I find myself wishing for more ventilation.

I bought a Golite poncho/tarp the other day. I'm going to try it out this summer and see if it's good enough to use for a poncho and shelter. I chose it because it's longer in back (to cover the pack) and corner to corner it is 9'11'' long, about the length of my homemade hammock.

So far I can think of one downside to the poncho/tarp (besides the fact that it makes a REALLY small shelter, less than I'm used to). Inside the poncho hood my head can get pretty warm. But that's just from trying it out around my yard. Perhaps in a rain it wouldn't feel that way. But I'm wondering if I wouldn't prefer to use a hat to protect my head and just use the poncho for protecting my pack and my body.