Well tankless can work for you but you would have to install at least two units. A single Noritz 931 can provide 6 gpm at a 70º temperature rise. With a 50º incoming water temperature that would give you 120º water. So if you pair up the 931 with another unit you can get 12 gpm with the 70º temperature rise. Now most people do not shower at a full 120º it will be mixed with some cold water so this would meet your need, as long as no one starts the washing machine while you are showering. If you want the ability to take the shower, run the wash and a couple other things at once then you can add a 3rd Noritz 931 to the system. Now whats the down fall? Cost! These units retail for $2100 each, then there is gas piping to take in account 3 units running full force will draw 750,000 btuh.
I would look into the Bradford White GX-1-TW-55S6BN This 55 gallon unit can provide you with 200 gallons of hot waster in the first hour of delivery. It recovers 84 GPH with a 90º rise. Here is the PDF spec sheet on the system. I have seen many homes with your type of demand with a pair of these units plumbed in parallel and where very happy. http://www.bradfordwhite.com/images/...eets/119-B.pdf