Tons of great charakters...

those which are by no means mistakes to bring them up to 60 are:

Madame, Dreng, Chaagur, Deliana, Deacon, Apo, Robar, Armiger, Tyrant, Royal Guard, Dhukk, Mausi Mage, Aox

If you are looking for an arena team, from the current settings I would suggest

Deacon, Madame, Chaguur, Robar....but you have tons of options there

Early game, the nukers are the only ones that have to be maxed to be effective. If all you have is Kael and Elhain, you want Madame, Psylar in stun set, Apothecary (or Deac/High Khatun, depens on the books you have). For whatever content you can handle right now, Madame and Psylar can work at 50 with speed, accuracy and tanky stats as a priority. Apothecary and Deac/HK will be your speed lead, so they need to be fast, I'm not sure if they need to be fully ascended to achieve maximum speed. Keep in mind that Apothecary also heals and Deac can help with controlling and debuffing. 

I've heard of Tyrant and Robar, but have no experience with them. When it comes to leggos, I am very careful about the ones I max, primarily because of books. Some leggos don't need books to be good, I max them out. The ones who do have to duke it out with other leggos. 

I'd work on champs with the most overlap in overall content.

Deacon is who I'd do first, he'll be your defense down/turn meter champ and you'll use him basically forever.

Then I think I'd want to go for a champ that has some sustain for your team which leaves us with a few choices in Mausoleum Mage, Tyrant, Deliana. I think I'd go Mauso here given that he's just an excellent all around support. 

Thanks! I already use Madame in arena, and she's great, but I was thinking Deacon and Madame might have overlap. I'm kind of seeing where they could work well together now, though. 

I agree with Deacon; I think he's got a lot of utility and would work well in many game modes. I forget about Maus Mage, but I'm stuck on Agreth in Doom Tower and thought he'd be good there. Probably good all around sustain. Lots of AoE dec. defense, but I wouldn't have minded pulling more support champs. I appreciate it.