
Mecha God Shepard vs Tengen Toppa Gurren Lagann: Make it happen, BioWare! (Of course, unless the Reapers have really stepped up their game, it won't be much of a fight... But hey, having Commander Shepard for a soul - or whatever the **** - has to count for something, right?)
In a similarly non-serious suggestion, here's the only Shepard cameo I have any interest in:

You're a weenie little Level 1 Ryder with all the combat potential of a slightly wilted lettuce. All that stands between you and one of these charging space gorilla thingies is a pistol that couldn't explode a gasbag. All hope seems lost. But then, out of nowhere, a resounding *BANG* as a blue streak comes flying from out of frame, crashing into the gorilla and sending it hurtling off a cliff--the blue streak in tow.
On the wind, whispering, you hear a familiar voice. 'Vanguards for life' it says, so quietly that you're left wondering if you actually heard anything at all.
And then no one ever comments on it for the rest of the game.