When Galactus is ko'd, does the herald keep any of the powers he received at the start of the game?

Also do the herald tokens remain in play after Galactus is KO'd?

If I pick a dou character and use the split mechanic, do both split figures then become the herald and share the powers and asset dial?

Also who would you suggest as his herald?

No, The effects are only in play while the trait is in play.

2014 Rulebook p. 17 "Game effects remain a part of the game only as long as the character continues to possess the power or ability."

No, you only apply effects to one replacement character.

Link to Wizkids ruling

In the day, I use to like Dr. Manhattan from the WMFF. With Running shot and +1's, he is a beast.

For modern age, off the top of my head the AvX Scarlet Witch at 150 becomes an amazing offensive piece with the Herald bonus to go with her powerful support capabilities to heal and buff big G.

I'm sure there are better pieces out there.

The AvX Professor X can get pretty nasty as a Herald.

OK, since I never looked @ the Herald Dial on him, does anyone know if Mystical Elf from the YGO set would be good? She has support, but has a special that free action roll d6, on 5-6 heal all friend adjacent figs 1 click