Oh NICE! I just got a new legendary hero! Four-star Fin, LETTZ GOOO!!
But is it better than my nine-star Kenju. I have enough EXP to get Fin to the same level, but I don't have enough dust to level Fin up to five-stars... Should I replace another hero in my lineup with Fin? 
Currently, there's no easy way to see how a newly acquired level 1 hero will fit into your team because we're limited to replaying our highest town. To evaluate a new hero's performance, you need to invest EXP ||(luckily with the new update, this is now refundable for 50 amber)|| and dust to level them up before making any real comparisons.
I'm suggesting implementing a practice mode to address this issue:
Battle Simulator Idea:
-Select your opponent's faction (teams could mimic those from the real conquest mode).
-Choose their levels (or make them all level 1 for simplicity).
-Select their stars.
-Choose your team from the heroes you own.
-Set their levels (this is crucial because if you are limited to your actual hero levels, you won't see a difference from a newly acquired level 1 Legendary hero in a team of level 30s; or, alternatively, make them all level 1).
-Select their stars.
-Simulate the battle.
-Experiment until you optimize your favorite team.
While implementing this sounds challenging, it would be a phenomenal tool for both new and experienced players. Wouldn't it be great to play around in a simulation without the consequences of spending your limited resources?
Let me know what you think about this suggestion. Does this remove the risk and, consequently, the excitement from the game mode?