Can't think of anything more to do to this, so for now this is the finished version.

As you can see, BIG cannon. But not that big, really. Think of the phaser lance in All Good Things, or even those huge cannon on the Negh'Var.
Also, an array of forward facing tubes, rather than just a couple, and the tubes much more prominent. I never much liked that quantum torpedoes seemed to be basically photon torpedoes of a different colour. I wanted mine to be bigger, obviously more powerful.

The aft end. Much bigger impulse engines, and twin aft facing quantum tubes. Also I relocated the shuttle bay. I never much liked 'drop type' shuttle bays. Either you park your shuttle on the doors, which seems like really bad engineering ("Sorry Captain, the door lock broke and the shuttle fell out."), or you have to have this expanse of floor space that you can't use. Either way, not good. So I put mine in what seemed like the most obvious place. Originally these were much smaller, but I resized them so that they could, just, take a Danube class Runabout. Runabouts turned out to be a good deal larger than I'd expected them to be!

There's a general absence of windows on my version of Defiant, because windows are weak spots. The only place you find them are in the hull sections containing the crew quarters. These are behind the phaser cannon, so difficult for an enemy to hit. And those sections are non-essential anyway, and would be empty during battle. The armour scheme is such that these sections are lightly armoured, but there is a big slab of armour behind them. So a hit there would blast the empty quarters before being hopefully stopped by the armour behind.
Note also phaser arrays dotted around the ship. These are 4 metres wide, as wide as a Type X, though shorter than the main arrays on a Galaxy class.

A close up of the Quantum tubes and a phaser array. Note there are also phaser ball-type turrets as tertiary weapons, because sometimes you don't want overkill. Like having 30mm cannon on a modern Destroyer because sometimes the target doesn't rate a 127mm main gun round.
Also, I figured that since the design was based on fighting the Borg, the thing to do would be to have as many different types of weapon as possible. As the Borg adapt to one, you switch to another - so you get as much time to strike as possible before your weapons become useless. To that end, I considered having the main cannon be disruptors rather than phasers. Perhaps Klingon weapons engineers assisted in the design of them? I also thought that the ship would probably be designed to allow the deflector weapon the Enterprise-D tried to be a standard feature of the deflector design. That would give the ship photon torpedoes from the nose probe launcher, quantum torpedoes, disruptor cannon, phaser arrays, and phaser ball turrets. Quite the loadout!

A close up of the cannon. The darker patches of the hull are thick slabs of armour - over the cannon, that armour is about thirty inches thick. Over the nacelles it's double that. There are other protective features, too - the bussard collectors have armoured grills over them, which means gas can still flow into them along the Y-axis of the ship but they're shielded if you go more than a few degrees off axis. The bridge is buried with hull structure all around it; the bridge dome is thick armour, and the glass ceiling dome is replaced by a big thick lump of extra armour.
So, some stats :
Length overall : 235.7 m (773 feet)
Breadth overall : 214.3 m (703 feet)
Height Overall : 71.5 m (234 feet)
Weapons : 4 Pulse Disruptor Cannon (4 fwd); 8 Quantum torpedo tubes (4 fwd 2 aft); 10 Type X phaser arrays (1 fwd, 4 stbd, 4 port); 1 probe launcher, capable of launching photon torpedoes (1 fwd); 10 Phaser turrets (6 drsl, 6 vntrl).
Crew : 135, facilities to carry 100 tactical crew with equipment.
So that's my take on the Defiant. Hope you like her!
Give a man a fire, and you keep him warm for a day. SET a man on fire, and you will keep him warm for the rest of his life...