You should mention where you are in different areas of the game, and post roster. This will help to get better answers. 

Someone higher up may say stick those in your vault and wait for a possible fusion, while someone at a lower level may suggest using them in a particular area.

Early on I used Rocktooth and Mystic Hand in 2 of the Keeps, untill I pulled better champs. All the others I have in my vault.

Mystic Hand is definitely one of the most useful in the Magic Keep, still use him in that dungeon, but just that dungeon alone. Lvl 50 is enough.

I personally keep all rares (and up) in the Vault, not as preparation for any future Fusion events, but just as collections, if I got a duplicate, I use them as books to the original champ that I got, if I already fully booked said champ then I use the duplicate as food.

I think you should think about how you would manage your "extra unused" champs from now on so you would have a system to quickly deal with them without any regrets.

Nothing really notable maybe except for Mystic Hand and Ragemonger. I suggest you keep working on your starter champ instead of spreading your resources too thin amongst multiple champs early on.

I used Stitched Beast in my early stages of Spider dungeon. Its deplete turn meter skill is very useful. I think it is not worth to 6 star, but build him at least 5 star, give it some good accuracy and try it for your self in Spider. Once you don't need it anymore and you got a duplicate of it, feed it to 6 star another champion

Out of the bunch the ones that stand out to me are :

Mystic Hand : great in Magic Keep as mentionned above. 

Rocktooth : Great against Spirt keep with his block heal 

Banshee : very good for AoE Crowd control, especially in a stun/daze or provoke set plus places block cooldown skills, can be used in dungeons and faction wars if you don't have better options, plus there have been all rare tournaments in the past , and maybe similar content in the doom tower where she will prove useful. 

The rest are situational and can be used but i wouldn't recomend taking any of them past 50 except for Banshee.

Save 1 copy of each Rare, Epic, & Legendary Hero you pull in your vault for Future Fusion's.

Hold on to Magus. You will need that one for Rhazin fusion.