I once did a side-scroller and, after watching Matrix, I thought about (but never implemented) a power-up to go into "bullet time" - maybe your character stays at normal speed but everything else (swinging doors, enemies, etc.) goes into s-l-o-w m-o-t-i-on.

Never played "Trine" (So I don't know if already had these features), but I always find a "run" button with shift very good for gameplay :). You could implement "double-jump" as well. Nothing new, but, always good for gameplay.