I think Warmaiden is your next candidate for levelling. AoE def debuff is so usefull in this game, you definetly need a champion that can do this. Bellower (who is another great champ) only has the small version of this debuff (30% def decrease instead of 60%), so for def debuff Warmaiden is your best option right now.

You don't need skillbooks for her (save them for other champs, books are really rare in this game). Instead you can book her skills with copies of the champ. Warmaiden can be farmed in campaign stage 9.

Other great champs you have are Skartorsis, Bellower and Sinesha. They are worth lvl 60 later on. 

I would not invest in Warpriest any further. You will get better healers.

Not really sure about Warmaiden being a 2nd 6* option. She'd only be needed for her Decrease Def and with enough accuracy and speed, she's perfectly capable at 5*. (But then I might only be thinking of arena and if so she can be better for dungeons and less fragile at 6*)

This might be the harder route due to books and more potion requirements, but I'd rather go with Sinesha or Skartorsis instead. They can be great supports, and provide better survivability for Athel with dungeon progression. 

Good luck!