I am enjoying this game a great deal, and I want to see it keep getting better.

This improvement applies once you are no longer trying to fill your party. Early on in the game, any character is good at DPS.

The major downside I see with the game right now is that you have 1, maybe 2, DPS characters in your party, and the rest either buff their DPS, or provide gold bonus. So you have 3 roles that characters can fill, Main DPS, Buff Main DPS, or Gold production. This is a great, simple system, but it won't hold interest for too long, and it will tend to boil down to the same ideal formations that are used again and again.

I think that more abilities would help to improve the game, and having charcters with more specific functions would be great. Most of the DPS improvement abilities (and gear) are wasted on most of the characters. Not to say that they shouldn't have DPS, but more to say that characters should have more consistent abilities and gear, and that they should be more relevant for a given role. Each character should have a easily identifiable role and sub-role.

The lumberjacks role is DPS, and he has a sub-role of Buff. The Princess has a role of Buff DPS. The Panda has a role of Gold Production. The werewolf is DPS. The Monkey and Detective are DPS and Gold Production.

I don't play with the Bushwacker, but his role would be Click Damage, and people do have buffs for him, but I don't tend to pay attention to that, if it was a little more interesting and streamlined, I might be more inclined to try it.

What I would suggest is a role and sub-role for each character, with more roles and sub-roles added. Any role can be a sub-role. Currently some of the characters have a little bit from every role, and consequently, in my opinion, are fairly useless.
Current Roles:
DPS
Click Damage
Buff DPS (this is seen as all crusaders or specific ones, like Sasha's ability)
Buff Click
Gold Production

Suggested New Roles: (There can be a great deal of creativity here!)
Healing
HP Buff
Buff Support Abilities (This would improve abilities that aren't DPS related, like healing or gold production)
Elemental Damage
Elemental Resistance

Healing is useless, as it currently stands, The witch doctor's ability just isn't good enough to make a difference. But if there where levels or entire challenge paths where your entire party was constantly being damaged, Healing would make sense.

Likewise, having HP isn't terribly useful. But if there was a challenge path where your party took increasing amounts of damage in one big chunk at the beginning of each level, you would have to have high enough HP for all your party members to survive.

So, basically there become different reasons to use different characters, and they it can make finding the right formation for a particular challenge more interesting and varied.

I hope I didn't ramble on too much, and that this makes some modicum of sense.

tl;dr - More abilities, slightly more focus = more fun and more challenges!