Currently I'm considering the following models:
1. pay for some additional units (cards) which would basically mean that some players who pay have additional strategies available (not necessarily more powerful, but rather more diverse).
2. pay to play more. Amount of duels per day would be limited, and thus the player's progress. The game will feature a ranking system with levels, so paying players could advance faster.
The second one seems closer to KoL model. Which now gives me an idea. I could separate the rankings system (leaderboard) from the player leveling system, so that leaderboard is limited to, say, first 5 matches each day, while you could still play additional duels to level-up faster. Although, leveling-up has no effect on the gameplay, it's purely cosmetic.
I'm still not sure if 1. or 2. would make more money, but making more money is not my primary concern. I just want the game to be sustainable and attract as many players as possible.
I believe many indie developers like me are facing the same questions themselves, esp. when they read that Supercell is making gazilions.
You want people to invest themselves in your game, and being multiplayer having more players online at once is only ever a good thing. Turning away players is counter-productive.