My concern and others seemed to have jumped on it is continuing to add heroes in our current environment. While obtaining and training new heroes is fun, playing with a usuable character that has never entered my lineup would be just as entertaining. Of the 60+ characters we now have, I'd say a good 20 have never been used outside of cannon fodder in the crypt.
Once you get past the vast number of useless toons, you have the double whammy of purple grind. I find it hard to work on underpowered toons due to the vast number of purple items needed to stay competitive with my current team. It seems to be a huge concern raised in multiple threads but never seems to be addressed.
Last but not least are the number of heroes still without legendary skills. This, paired with my first concern, can easily bring old toons to the forefront. Look at the benefit it gave to heroes like Dwarven Archer and Bone Dragon.
I feel the community would far benefit from these additions before more toons were added.