(Please excuse me for making assumptions about your intent and rolling with it, but I got carried away.)
It sounds to me like you’re comparing Trust Levels to a traditional leveling system.
That’s not exactly what Trust Levels are. Whereas a system like Khan University or Duolingo (or just about any MMORPG) uses gamification as a carrot-on-a-stick motivation, Discourse uses it only as positive reinforcement. It doesn’t tell you to like a post, but if you do, you get a high five.
The Trust Levels aren’t so much there to coax your users into becoming more active members of your community as they are there to acknowledge those members who choose to stick around.
This is how I see the Trust Levels:
Basic: You visit on occasion.
Member: You visit quite frequently, but you consume a fairly small amount of posts, specific to your likings.
Regular: You visit obsessively and you consume a large array of posts because you have a vested interest in the community.
Leader: tbh this one confuses me. It’s a mod, sort of?.. (might start a new discussion about this)
I don’t think any amount of intermediate levels between Member and Regular could motivate a Member to become a Regular, nor should they. A Regular is a bit of an odd (yet cool-as-ice) duck. They’re a minority, and rightly so, because if more than a small minority is obsessively active in your community then, sir, you’ve started a cult and the FBI would like to speak with you.