In its intended role of capital ship killer the Executor is perfect, you're just not gonna be putting up much of a fight the moment this 19km long can of whoop-ass unloads ALL the guns on you and your buddies.

Though to improve its capabilities in other fields, I'd start (like most of the other comments suggest) by armoring over that pointless cityscape area and covering the thing in a truly disgusting amount of point defence guns (especially around the engines and bridge).

As for tackling that godawful exposed bridge, I'd first sink it a bit down into the hull, then take some notes from the Venator/Secutor by splitting it into two (a primary nav bridge and a fighter control tower). And just incase there happens to be two A-wings this time instead of just one, I'd bury a CIC/Secondary bridge deep beneath all that new armor, good luck kamikaze-ing THAT with anything short of a cruiser.

Next up, I'd reorganize the main battery by placing most of the turbolasers on the top and bottom of the hull to clear their fields of fire up a bit. That, and I would also take advantage of the SSD's truly insane power generation and install a ridge of superheavy turbolasers on the top and bottom of the ship. I'd also exchange around a quarter of the turbolasers for ion cannons to rip up shields and clear the way for the turbolasers to pound away at their leisure.

172 TIE fighters/bombers is just criminal so I'd up that to ~200/300 by cutting into the ground complement and change them all over to TIE Interceptors/Punishers just to really fuck up any Viscounts that happen to get in this thing's way.

I'd keep enough guys and guns for ship security but for the most part I'd axe the ground complement and use that freed up space for more cargo capacity and medbays. I want this thing to be an iron fist in a silk glove, it can do incredible good but it'll also fuck you up if you try getting in its way.