Titanic II should be relatively easy to build, being a small ship, by modern standards (56,000gt).
The engines/pods are all of a standard design. There's no LNG capability. I'm guessing that the external apperance and internal decor will be the biggest challenge for Tillberg. Modern fireproof materials will have to be used instead of all that natural wood. There will be some extra fire exits too, in some public rooms.
Surely, even even 'steerage' cabins will have private bathrooms.
There will of course be no atrium, no pools or hot tubs on deck, no balcony cabins. Palmer even said no TVs and no Wi-Fi.