A380 all the way if you have a 450-550 seat layout. You can do e. g. 12/98/440 (or whatever you choose) on the A380.
That might be a bit over the top...I always feel like I'm wasting space if I just cram in half the seats that the aircraft is AE-certified for.
the 748i isn't bad either, but it's smaller, so likely won't work with your current high-density layouts. If you're worrying about turnaround times, don't. The FF of the 748i offsets the difference by a wide margin and the A380's time is only slightly longer.
Okay.
If you wait a few game years, you might want to use the A350-1000. It has a good runway performance, though is a bit smaller than the 77W and definitly smaller than the 744 so you'd have to up the frequencies a fair bit.
The A351 has a damn attractive range...
I might just go for both the 350-1000 and the 748i...with efficient seating in both.
Why trim it down? Keep adding gradually to the fleet and keep gradually adding routes and you can't go too far wrong
If there's no empty routes to fill, I don't expand. If someone jumps my routes and cuts prices too dramatically, I leave. I'm not playing that game, no way, no how.