I notice there is somewhat of an access imbalance between Mario and Richter. What I mean by that is Richter can do nearly everything Mario can, while Mario has few things Richter can do.
Both Mario and Richter can defeat enemies by landing on them.
Only Richter has a default weapon.
Both Mario and Richter can collect coins.
Only Richter can collect hearts.
Mario and Richter can break bricks.
Only Richter can break candles without requiring a powerup.
The balance is too far tipped in Richter's favor. Likewise, the delay to change characters really incentivises someone NOT to bother with mario unless necessary since rapid switching is slow and tedious. (I also felt switching in CV3 was too ungodly slow.)
I don't see why Mario cannot collect hearts that would add to Richter's stock, especially if Richter can collect coins that add to a shared stock. Likewise, Mario could also be able to break candles by landing on them as if he would an enemy, causing a bounce. This could lead to some interesting one time chance platforming segments as well. Furthermore I think Richter should NOT be allowed to land on enemies to kill them, as that is Mario's only method of default attack, not to mention his own signature move, and Mario has no whip to compensate. Likewise the whip is Richter's signature move, so it stands to reason Mario does not have it.