First time posing but this Idea has been bugging me so I guess I'll suggest it. A undertale/Deltarune colab. So here's the idea:

Undyne - Anhur
Undyne is a pretty good idea for Anything due to her use of spears in the Undertale game. Anhurs shifting sands can be a Multi Spear inpale from the bottom just like Undyne is known doing. The other abilities can involve a blue spear she uses.

Alphys and Mettaton - Sylvanus
In Undertale Alphys is the creater of Mettaton so why not put them as a duo. The basic attack would be them throwing bombs as Mettaton does. The Verdant Growth can be a thrown stealth bomb that was in a Mini game with Mettaton in Undetale. The Ult can be Mettaton turning to Mettaton Neo and slamming the floor.

Sans - Baron Samedi
This one gave me an interesting concept but it all worked out well. Baron's staff will not be in play with this skin because Sans doesn't have one. Vivid Gaze can just be Sans signature Gaster Blasters ( With there Iconic Sound ). Consign Spirits can be Blue bones that come out of the Ground. Wrap It Up can be a Blue Heart that is thrown to keep them stuck. And Life of the Party can be the Fire Door that Sans uses in the Undertake game.

Susie - Heimdallr
This one was an idea that kept me thinking but this is what I got. Piercing Sight would be and Axe like Susie uses in Deltarune. Gjallarhorn would instead be substituted by a constant Redbuster move from Deltarune. The Bifrost would just be reskined as a Ruby Heart. And Through The Realms would be Susie Lunging forward with her Axe and launching gods threw different areas of Deltarune.

Jevil - Nu Wa
This would be the last one of my suggestions. Mysterious fog would be the same except that the the ground below the fog would have a deltarune logo. The basic attack would consist of Hearts, Clubs, Spade, and Diamond because he uses all of that in the battle against him. Clay soldiers would be him summoning rubber duck like creatures that represents the ones he uses in Deltarune. Shining Metal would be a big Spade due to his use of them. Fire Shards would be him going up in the air and hitting every god with a explosive Scythe.

This idea but probably never happen but I wanted to leave this out there so i wouldn't keep it to myself cause you never know with these things.