Tom,
I know your supposed to bring 3/4" lines for water to new bathroom, then branch off in 1/2 inch.

But, but inlet water to water heater comes in at 1/2" and feeds my bathroom and kitchen on half inch, at least for the hot water.

I am tappin off the water inlet to the water heater, and also the hot water out for the new bathroom.

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"? Thanks.

What kind of water heater younar using? Tankless type or tank type. Water Heater are designed with their port, ussually calculated for their capacity. I believe you should stick to 1/2". by increasing your pipe to 3/4" doesn't help, it only dampens pressure, reducing it. I think its not a problem with 1/2", in case you wanted remodelling, you can use circulating pump to increase your output, in case you are using a tank type. Have a nice day.

It's a regular 30 gallon tank with 3/4" inlets, but it was reduced to 1/2". I think I will stick to the 1/2". Thanks.

"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