In case you see this post and think TLDR just go to my sig and see the 2011 BYOAC UCA link and go to the BYOAC Wiki and read everything under Controls.
8 buttons... I think that's seen mostly on the console sticks (SFIV type deal) which if that's your deal fine but doesn't have a lot of arcade applications. I don't like playing any console beyond SNES on my cabs and even those are limited. Console games, for me, are best left to the realm of the hand held controller and the couch.
As far as controls, without stepping on too many toes, it has been for quite some time that when doing an all-in-one panel it's commonplace to add a fifth (or even 6th) joystick. These are usually 4 way sticks. They have a diamond shaped restrictor that allows the player to slide across the corners only ever going in the 4 cardinal directions. This was necessary for gameplay on Pac-Man, Donkey Kong, and scores of other awesome games. If you played with an 8way stick you'd hit a diagonal and your player would freeze or go the wrong way.
Also, since you already had 5 controllers why not one more, you'd see a TRON stick (8 way stick with a special restrictor and a flight stick handle), and oh why we're at it we can't forget the trackball for missile command and the tempest spinner! This is commonly referred to as a frankenpanel. Do a search and you'll get all kinds of hits for that term. Very educational.
Anyway, there is not a good combination solution for a trackball and spinner. Some folks will use a rotary joystick as their spinner but this can lead to poor player performance and just isn't that fun, some folks disable the Y axis in their trackball using only the X, also a copout. There is no great substitute for a spinner; however, tons of people (the majority) never have spinners and are perfectly happy not playing those games. I didn't have a spinner for 8 years and was fine. Of course you couldn't pry my spinner from my cold dead fingers now but gaming tastes evolve over time.
Ditto the trackball, most everyone will agree that if you don't have one then don't play those games. You aren't missing out on that much, some, sure, but not that much.
So for those two controls you need to decide if you want them or not. If so great, if not, don't get them just to have them.
As for the a fore mentioned fourway sticks there are some great solutions coming out to remotely switch your 8 way into a 4 way based on what game you're playing. Ultimarc is selling one now, I'm sure other retailers will soon follow with their own version, but best of all, you can build one yourself. Check out the
wiki. This saves you from having to add a fourway stick at all. Is it 100% arcade accurate? No, but nothing about building a multipanel is so don't sweat it. And that TRON stick, you don't really need that anymore as you can build that right onto that rotatable 4way/8way stick using this
mod. There currently isn't a commercial version of that but I expect to see one crop up in the next year or so.
Then you have top fire sticks (taken care of by TRON stick mod) and rotary sticks. Rotary sticks are great for games like heavy barrel and ikari warriors but honestly like 2% of gamers have these on their cabs... but if you want them you can mod your stick to do that too!
The last thing is admin buttons. On your CP you'll need P1-P4 and likely C1-C4 (coin) unless you're putting the coin buttons on the coin door where they belong. Some folks have buttons for exit, favorite, list, pause, save state, load state, volume up, volume down, and lord knows what else. That's cool and if you've got room rock on. I prefer just to have exit. To enter a game I use P1 so that's a dual use. I'll usually make P1 and Exit together Pause but that's it. It all depends on what you want.
If you want to check out some awesome user approved builds go to the 2011 BYOAC UCA link in my sig and cruise through all the different category links to see what's out there.