That's a good idea - one potential metric could be something like "average zaps per note".

Another way you could fix the Follower problem would be to make following someone cost some nominal fee in sats - could be pennies but would rapidly become cost prohibitive for bots to follow everyone under the sun like they do now, much in the same way that physical junk mail has costs that limit their reach. One problem is bots would then predominantly only follow prominent people. In that case, one could make the fee adjustable so a prominent person could charge more before you can follow them.

Further to this, for individual users the cost can sometimes cancel in an "I follow you, you follow me" scenario.