A coupe has two doors.

A sport coupe has two doors and is small/light weight like a Miata, Corvette, or Ferrari Modena. It may have a vestigal back seat like a Porsche 911 or jap-spec RX7.

A GT coupe has two doors, is heavy and luxurious like a Ferarri Maranello, Mercedes SL or Aston Martin Vantage. It may have a vestigal back seat like an Aston Martin DB7 or Lexus SC.

A sedan has either two or four doors and a reasonably functional back seat. For example, a Chevy Monte Carlo is a two-door sedan.

A sport sedan is a BMW M3 or any of its assorted imitators.

Interpretations/corrections are welcome.

The Altima Coupe ain't gonna happen. The last front wheel drive 2-door that Nissan has sold was the '99 Sentra a.k.a. 200SX.