Increase 1/2'' copper water pipe to 3/4''
If you are looking to increase the water flow to the fixtures in your house but do not want to re-pipe the whole house, you could increase the supply pipe from the 3/4" inlet to the water heater. Tee off any of the individual supplies from the new 3/4" supply with reducers and the final pipe into the hot water tank could stay 1/2". This way you would double the supply water available to all the fixtures since the hot and the cold will have individual supplies instead of a 1/2" supply supplying both the hot and cold. Also, remember that almost all sinks and toilets use a 3/8" supply and only the tub and/or shower and the laundry tub use a full size pipe.  Doing it your way would be fine.