The new ship would have to comply with all the current SOLAS (safety of life at sea) conventions. The loss of the Titanic was the catalysis that started the move towards increased Safety. Since then (mainly reacting to numerous disasters) the conventions have been enhanced over time.
Technological advances will also alter the ship. The Titanic used riveted rather than welded steel for its hull, this reduced the strength and increased the iceberg damage. Modern ships use diesel rather than coal, most of the funnels would be decorative (a tactic used by Disney Cruise Line) and the engine noise would be different. The RADAR, communication equipment, helipad etc. would all change the look of the ship.
The onboard facilities would have to match modern expectations (such as larger cabins with en-suite bathrooms and air conditioning,most cruise ships have cabins with balconies. The very large first class suites (with extra accommodation for servants) may need to be reworked to meet price targets and get enough customers.
In the end I think this will end up as a modern ship with a similar outline to the original Titanic, with some internal areas looking similar. I would like to see a (very) accurate replica, this would be interesting to visit for a short period of time but would be difficult to sell to customers (especially in 2nd and 3rd class).