The Voyager-class was the altered design of the standard Intrepid-class that was commissioned in 2405. It was a variant of the Intrepid which included two additional nacelles (one above the shuttlebay, and one above the bridge). Those nacelles, along with the new class-10 warp drive, allowed it to reach warp 13.
The ship was classified as an explorer/battleship, similarly to its predecessor. The forward torpedo launchers are now supplemented with quantum torpedoes as the type-10 photons and tricobalt devices (not standard issue on the Intrepid-class) become standard issue weapons of the class.
The type X phasers that used to be part of the Intrepid's main phaser array were replaced with the type XII phasers used on the Sovereign-class. Also included are emergency phaser cannon launchers, lined up along the main phaser array, which fire phaser cannons, similar to those of the Defiant-class, in case the ship's phasers are disabled. Two torpedo launchers have been added to the primary hull, one port and one starboard, similarly to the Borg enhanced Voyager in the episode "Scorpion, part II".
The multiphasic and multi-spectrum deflector shielding of the Intrepid-class remain, with the addition of regenerative shielding and ablative armor generators installed. A key thruster has been added to the bridge nacelle, which can be damaged if a concentrated phaser strike manages to get through the shielding. The result is that the ship will careen out of control and the other thrusters would not be of much help.
It is not known why this design was not implemented in any of my fanfics, but it could be soon.