I'm currently level 70 and I have a lot of the heroes but not all of them leveled.
What should my end game composition look like so I don't waste my resources on heroes like Victor and Ember for end game?

Mostly used heroes are Gale,Garrick,Jasmine,Drago,Leon,Riley,Dokras,Arachna
I suggest u to focus on these but if u have time dont neglect these heroes-Ethera,Ulfang,Musashi(for raids and HoL),Ariel,Gizmo,Serafine,Osiris,Tholin,Tanya..
Keep at least 10 fully skilled and levelled heroes for mobility(Arena,Crucible and raids) when u reach 80 u ll have a lot of time to focus on other heroes cuz it gets harder to progress )

Leon, Drago, Jasmine, Garrick, Ethera, Gale, Flora, Slim, Gizmo, Leah, Mortus, and Urestag are the ones I would really concentrate on.

The thing is, once you get to level 90, you'll be able to level up characters like Serafine, Wraxius, Ariel, Dothras, etc fairly quickly so it won't make much of a difference. I would concentrate on the other characters first though.

I don't know enough to say about Ezio, Cara, and Tanya to say but they all look like good ones.

Are there any example epic arena compositions I should work towards?

For EA, you need to have enough heroes to configure three good teams (so minimum 15). So minimum of three solid tanks, though you can use some half-tanks to get by. Most of the heroes already mentioned will comprise your EA teams. Three main tanks are Drago, Leon, and Elric for me, so I worked on Elric even though I had abandoned him previously in the mid 70s. After that, you want to understand team synergy and put together teams where heroes enhance the abilities of others in that group.