You should post your full roster. Much easier to give advice that way.

At first glance, your CB team seems pretty good. Kael is definitely the weak spot on that team and needs replacing, Fayne would be a good replacement. Swapping Ninja for Geo may also help, though that's a tough one to compare.

Arena definitely needs work. That team doesn't make any sense together. Sethalia and Vogoth are meant for go-second teams, while Kael and Ninja are meant for speed nuke teams. Without seeing the rest of your roster it'd be hard to answer, but swapping Kael for Vogoth and Ninja for Scyl could definitely work as a go-second team, though it's very gear dependent.

My heroes

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Okay. So a few general points:

1. Other than Faction Wars, nothing in this game is intended to be done with champs that are not 60. Any challenges you're facing will be almost entirely due to having teams running sub-60 champs.
2. Stop upgrading common and uncommon items. They are uniformly trash. Do not waste silver on them. Also, be very selective about upgrading anything that is less than 5*.

For CB: Your posted team, swapping Kael out for Fayne, should be easy two-key NM, possibly even 1-key (with gear ofc). One thing to note - Sepulcher Sentinel is your stun blocker, so make sure she is on a 4:3 rotation (~224 spd), and make sure she uses her DEF up either on turn 2 if she is last, or on turn 3.

For Arena: You're much too early in the game to be caring too much about it. At first glance, though, I would expect something along the lines of High Khatun lead, Seeker, Ninja, and Magnarr would be your best bet. You will need some decent speeds though - HK around 250, Seeker around 220, Ninja around 190, and Magnarr around 160. 

To answer the initial question: your next lvl 60 champs should be Sepulcher Sentinel and Skullcrusher.

Leech is your way to keep your CB team alive. All members in your team need to be lvl 60 and have Giantslayer or Warmaster skilled as tier 6 mastery. That should be your next goal. After you've done that, you could either lvl up a replacement for Kael in your CB team or Scyl for your dungeon progression.

Clan  Boss:

-  Sepulcher 

-  Fayne

-  Kael

-  Skullcrusher 

-  Geomancer

Arena:

-  High  Khatun

-  Old  Hermit  Jorge

-  Alexander  or  Warmaiden

-  Ninja