| "Question is: should i run a 3/4 line off the 1/2" mains, and then branch off into 1/2" again at my fixtures, in anticipation of re-plumbing my mains in the future to get a 3/4" feed? or just run my new bathroom all in 1/2"?"
Nick was correct when he advised, "I believe you should stick to 1/2". by increasing your pipe to 3/4" doesn't help, it only dampens pressure, reducing it."
He was not when he said," i think its not a problem with 1/2", in case you wanted remodelling, you can use circulating pump to increase your output."
You can not increase volume by adding a recirculating line. You simply can not get more volume out of a system that you put into it.
There would be no reason to increase to 3/4" after you've choked it down to 1/2".
Code mandates that no more then three fixtures be supplied off a 1/2" branch, However my 1950's house is supplied with 1/2" and it's not been a problem for two of us that live here so I'm not condemning it. Good luck, Tom | 
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