I haven't really encountered any issues with controllers not having enough buttons myself. Any more buttons would only make it more difficult to control, I think. If there's a need for more buttons, perhaps the devs need to make controlling a character less complicated. The only games that I can imagine would truly need it are simulations or strategy games, and the best way to play those is with a mouse anyway.
I would rather see more inventive use of buttons like the triggers of the GCN. The way they used that extra 'click' at the end of the push in Super Mario Sunshine to make Mario stand still instead of run while using FLUDD was brilliant. Also, better D-Pads. I'm looking at you, Microsoft.