What if we included some characters we made? Like common monsters (the ones you battle randomly)
We could each make up like, say...4 common monsters from each of us with their own looks and personalities. Plus an amalgamate of 2 or more of the characters we made, and maybe 1 more major character we each made. WE being you, me, and Enderwings and plus whoever made Sans and is suppose to be doing the major characters (and whoever else might join in to write the story better if we don't figure it out ourselves)
That way we have our own sorda designs in it as well. That would mean we would each have 5-6 (depending on if an amalgamate is made) characters of own own making in the game which would partake in the pacifist and the genocide and the Zeta reboot Sans inflicts. Not only would this make it more difficult, but it would also make it specifically OURS. It would kinda be like "we spent a very long time on this game, and seeing our characters come to life is our reward for that"

Since the battle with Scarf Mouse at the start of the bridge and Narcat Jester at the end are meant to be taking the place of the battle with Papyrus in ZetaTale...what if you made that 1st, as like battle type thing? Then we can see how that goes over?

Two is that there's a lot of...implied characters...like all of those in the hotel rooms and the one in the house in Snowdin that seems way to happy about you knocking
We need to figure out how we're going to approach THOSE characters. Now, I had this thought...since those characters would only be out of their places in the Zeta reboot and not through normal Pacifist or Genocide I have come up with 3 options.
1: Use some of the deleted characters from that video I sent as the characters which take the places. This would explain why they're not seen in the actual normal game of Undertale but are in the files still.
2: Use our "major characters" to fill the gaps so that we don't have to figure out how they fit into the story line or make as many sprites of them. This can also be used for if the character looks like they belong in another part of the game, as they were simply staying at the Hotel. That...doesn't really work for the Snowdin house though, so that one will at least somewhat have 2 blend in.
3: Wait till we're near completion of the game and have put out teasers and such so people are interested, then do some sort of contest saying we need to fill this certain number of slots with characters and if you'd like to be featured to post you character and what their attacks are as well as possible lines of dialog while fighting. Also specify that if they're not already designed as Outertale/Zetatale characters that there will be small redesigns so they fit the universe better.

Ask Blankw0lf (Tumblr) about using his OuterTale sprites
If not, ask this person: link
EnderwingsThePony (DA) doing sprite art
Ask CandyDude (Youtube) if we can use their Outertale opening
Ask SpaceFall (YouTube) if we can use their Outetale songs

Once we get a few of the fights programmed specifically for ZetaTale send to Mairusu (Youtube) to play