Oh, Sans would be pretty cool, I think he could work as sort of a... Glass cannon, so to speak. Cool, unique abilities and great power, but, low defense and health. Although, this may be controversial, but I was never the biggest fan of Sans. Of course, there's nothing wrong with him, he's a great character, but I just found him... Too laid back and silly, you know? On the other hand, I would love if Asriel joined the party, as unlikely as it may be, who wouldn't want to see Ralsei(Who's name is an anagram of Asriel, something I found out completely randomly, and his apperance is also somewhat similar to Asriel's!) and Asriel fighting alongside eachother?