How would making a sequel contradict the message that resetting everything back underground is a bad idea? It would be a completely new game on the surface with a completely new plot and characters. I think the SAVE power as a tool that can be used for good and for evil.

I don't think the player not being Frisk is that big of a deal. I always choose the Canon name for the protagonist in these games anyway (Like in the legend of zelda or Pokemon I always name Link as Link or Red as Red, I never thought of it as my story but theirs). Maybe this time there is not a "name the player character" screen and from the get go the character's name is Frisk with no way to change it (I would prefer that in lots of games actually). We're not in the Underground anymore so no more fallen child or you could say they left after the ending.

I think I hate that storyline. It is the low hanging fruit to make cheap and uninteresting conflict.