You don't need ANY extra buttons besides your 6 buttons for each player and your start buttons. Everything else just adds to panel clutter. Any admin buttons are best put inside the coin door, or simply as shift functions off one of the player start buttons (you do know that you can assign things like TAB to "Player 1 start+Player 2 start" so it only tabs out when you hold down both buttons).
Spinners and trackballs can use the buttons off either player one or two. They DO NOT need dedicated buttons. If you want actual mouse buttons then wire a couple of your other buttons as the mouse buttons (Player 1 and player 2 start are good choices for mouse buttons).
As for the Neo Geo button. Have you extensively played any 4 buttons Neo Geo games in the arcade to an extent that you need the button layout to be the same (only the fighters are 4 button), if so, then go for the Neo Geo layout, otherwise it is just extra cost and extra clutter for no real reason.
The best "secondary" control to add on a budget is a 4-way joystick, or even better a pair of them (one for each player, as they are also better for puzzle games than 8-ways are). They go well directly to the left of each standard joystick, and sharing the same buttons as them.
After a 4-way (hundreds of games use 4-ways) the next best control to add is a trackball (about 60-80 games use trackballs, or at least play well with them). Next up would have to be a spinner (about 20 games).
Of course the sky is the limit when it comes to different controls. The specialty ones are almost endless, 360 wheels, 270 wheels, several kinds of yokes, steering harnesses with buttons, all kinds of shifters and pedals, several kinds of rotary joysticks, 720 spin/joy, Xybots sticks, analog sticks, Wizard of Wor 2x4-Way sticks, positional guns, light guns, up/down spinners, several kinds of top fire sticks, Vindicators controls, Assault sticks, 49-way sticks, several kinds of dance/"Stompin" pads, keyboards, turntables, pressure sensitive buttons (original Street Fighter), several kinds of boxing/arm wrestling controls, 2-way joysticks, 270 spinners (many pre-80 games), levers (Galaxian cocktail and others), bicycle pedals (Prop Cycle), holographic guns (Golly! Ghost!), Frontline spinner (different from others), bowling ball (Hyperbowl), Pinball plunger, roller (Major Havoc), and about a zillion others!