As it is. We only have characters that either make money faster, gain energy faster, or stamina faster. But other than that there is no difference.
One interesting dynamic would be to swap which Stat you need 2 stats instead of one to buy. Leaving energy as difficult to get for only one character type.
A fighter could have to spend 2 to upgrade defence.
A hustler could have to spend 2 to upgrade attack.
A runner can have to spend 2 to upgrade health.
A healer could have to spend 2 to upgrade energy.
However it works out, you may find you need to change the order of who pays what for what. But mix it up for a little diversity.
The different character types would have more specific strengths and weeknesses compared to the other types. I think it would be alot of fun .