having them pop up would be great but since there's this adventure mode, they might just pop up in that along with every other mk character known and maybe you get a chance to fight them and if you win you unlock them.

If the secret character's sat/stood in the background or did a kata while you were fighting, that would be cool. Remember in MK Reptile was in the arena before the fight. Maybe you see the secret character before the fight with a clue or something.

2 cents.

Great ideas. I can see it happening right now in 3D and great graphics. And murdoink, that brought back good memories.

The flashing idea is good to, maybe in a level where theres lightning, for a split second, the lightning could flash and you could see a human figure, or maybe a shadow. Glowing eyes would be cool to.