I think I would go in a very different direction.
The overall shape of the ship and its parts would be almost identical to the TOS Enterprise, but everything would be larger. It would be as if the Federation took the original ship, tore it completely apart, recycled its components and rebuilt a larger ship to replace it. I would also give it a new class name, like "Magna Carta" class or "Liberty" class, to continue in the same vein as the "Constitution" class.
I would start with the cylindrical nacelles, and the roughly cylindrical secondary hull, and make them all longer. The nacelle pylons would be a little bigger and fatter, but not back-swept like TMP. The big dish would not protrude from the front of the secondary hull, but also not be flush with it either. If it had to glow, it would glow more softly like the Enterprise-E's dish glowed.
I like the fatter and swoopier JJ Abrams "interconnecting dorsal" neck, but without all the little details. No torpedo banks in the neck. Secondary hull torpedo tubes would be located either in the base of the nacelle pylons or the bottom of the hull (bomb-bay-style). The hangar deck and service bay would be longer and larger, as the overall secondary hull would be substantially longer and larger. There would be a small micro-saucerette at the top of the connecting neck underneath the primary hull's impulse engines. This structure would be large enough to house a TNG-style "battle bridge" and "ready room" command deck, but would not be bulky enough to obstruct aesthetic views of the lower primary hull from the rear
The saucer would be largely the same in overall shape as the TOS Enterprise, but the aft upper saucer dome would grow its own hanger deck. The impulse engines would be split to closely flank the saucer's flight deck. As with TOS, the upper and lower saucer domes would glow. The lower saucer would also house at least one torpedo tube. The phasers would be pure TNG, though not as visible. The saucer would also house small, retractable warp nacelles which could only be used after separation; they would typically be concealed. These would give the saucer limited independent deep space and combat capability.
The saucer hull would have a very similar texture to the TOS Enterprise, but it would have some more portholes. Just don't go overboard like TNG. The overall color of the ship would be more silvery like what Vektor
did with his re-imagination. The "air force" style block lettering on the hull would be the same as TOS, and in the same general arrangement, except that flight deck fantails would include "ENTERPRISE".
I would want the upper saucer to be shaped much the same as the original, and also the shape of the outer saucer rim, but I'm not sure if I'd want the undercut around the inner-rim.
The only major change overall would be to replace some of the colored wall and "support girder" structures with a more metallic finish. Color would be toned down but not eliminated. The overall bright look and texture of the interiors would remain largely the same, with some TNG influences. Control consoles would be only somewhat more sophisticated. Switches' levers and knobs would still be used, but so would touch-screens.
The TOS bridge is the best of all of them, in my opinion. I would want the same general arrangement and many of the same specifics. The refit bridge would be overall the same general design, but maybe a little larger. I like how the TOS consoles along the outer bridge ring flowed, and how the flat-panel displays flowed up from there, and how the overhead displays flowed (but were distinctive) from there.
Chairs for officers and crew would be made so that they would "attach" to the deck with the flick of a switch, but they could be easily decoupled from the deck for removal. Chairs would also include padded safety restraint armrests. I suppose the best (if somewhat cheesy) example of all of this would be the Excelsior bridge chairs
as seen in TMP3. (Actually, the Excelsior bridge
had many design cues that would've made a great refit Enterprise bridge.)
The Engine Room, Sickbay and crew quarters would've been much the same, but bigger and with a little more ENT-like metallic decor. The Jefferies Tubes would've been more like a cross between TOS and the TNG Enterprise. I'm not sure what amenities the crew would have, but civilian scientists, specialists, observers and dignitaries would be along for the ride. The whole TNG idea of families would not be part of the mix, though.
Hand devices and sidearms would be sleeker than TOS, with some TMP3 influence
Interior corridors would be bright and functional, like TNG, but with a little metallic touch like ENT.
Uniforms would be a cross between TOS and DS9. From the time I first saw TOS, I thought the long-sleeve T-shirts and black trousers and boots looked so cool as well as being practical. The same for DS9 jumpsuits. I'm not sure if characters would use commbadges or hand-talkies.
Shuttlecraft would be sleeker and less bulky than the Vulcan courier. The Enterprise would carry at least two runabout-style ships, several shuttlecraft and some smaller fighter-like probecraft. There would also be greater use of robot probe-missiles and some robots for servicing the Enterprise. A landing party crew might be supplemented with a robot.
Overall crew complement would be at least 500.