Galaxy-sized dreadnought with multiplex ablative armor, harmonic shields, CIWS arrays*, multidirectional pulse phasers, gatling torpedo launchers, tricobalt devices, and impulse-only parasite craft** to allow for assaults on multiple vectors.
Improved armour and shields are
defensive measures having them doesn't make a starship a 'single trick pony' like a battleship or "dreadnought"
CIWS are manifestly inferior to both existing options and basically redundant given the existence of shields.
There's nothing in canon to suggest that pulse phasers
can be multi-directional and the fact that the
Defiant's dorsal and ventral phasers (which are multi-directional)
aren't pulse phasers somewhat argues against that.
The only ship to carry tri-cobalt devices -- and use them against a
stationary target -- was another multi-role starship, not a dedicated warship. Which suggest that they are provided on
mission-specific not
type-specific basis.
The Galaxy class was already fitted for Danube-class runabouts, so less capable impulse-only craft that would therefore have
vastly inferior shielding seems counter-intuitive.
And then put someone like Edward Jellico in charge of it.
I disagree.
Jellico was a rigid, pompous control freak who would order most of the improvements ripped out as "unreliable" or run the crew the crew into the ground before the first engagement "fixing" imaginary flaws.
Someone like
Picard wouldn't necessarily be appropriate, but someone like Ross or Sisko could work.
*Small, rapid fire weapons designed to take out incoming weapons. Equivalent to the Darwin kids' "aggressive" immune systems.
As noted above, essentially pointless, especially by the TNG era, given the existence of shields (arguably the earlier ball turrets
are CIWS or at least a hybrid of CIWS and autocannon).
**Warp engines have to be pretty bulky. Strip them out, and you could have Defiant level phaser power in a much smaller and sleeker unit. So in addition to all it can do itself, this ship could put maybe six or eight Defiant-equivalent spacecraft into play, either to fight alongside them or to protect a small area.
I've already touched on the suicidal/criminally negligent aspects of that idea above, so I'll just reiterate that the runabout is about the largest auxiliary craft which could "parasite" on the Galaxy-class or similar.
YMMV to what extent they would actually be helpful.