The more honest way to monetize games (especially skill games) is to be generous with what you give out for free and monetize things that are superficial or provide variety without providing advantages. Valve's Team Fortress 2 is the prime example of this. People pay lots of money for decoration (e.g. hats), which have no in game consequence (besides looking cool) or they pay to get hold of certain weapons that provide variety of gameplay without necessarily making the game easier (e.g. the sniper's bow and arrow is roughly as powerful as the rifle, and still requires a lot of skill, but once you see someone else with it, you do want to try it out).

Fundamentally, the free experience you provide in your game has to compete with any other activity the potential customer doesn't have to pay to do. The potential customer could play any of the games they already own, or they could go for a walk or surf the Internet. Either you have to be generous with the basic free experience or you have to use sneaky tricks to get the customer to play. Being generous with the free experience also means not giving paying customers an advantage in competition. No-one likes being beaten by a worse player who has paid to be overpowered.

This is a really tricky subject though. For example, providing a demo version of a game and charging for the full game is definitely not sneaky, right? It also seems reasonable to provide a time trial of X free days before you have to pay for the game to carry on. That doesn't seem like a bad thing either. What if you wanted to be more generous so the customer could keep playing your game as long as they liked (as with a demo) but they also got a taste of the full version of the game (as with a time trial)? Well, I guess you could have a demo version of the game that also gives you a free 10 minutes of the full game every day. Is that okay? It seems okay at first glance, but, it's also kind of close to being a reinforcement learning manipulation. If you made the free time period a little bit random, or made it happen at a random time during the day, then you'd have something definitely manipulative.

I think there are new fair monetization strategies to be developed regarding cooperative play. Games like Clash of Clans make use of cooperation to inspire purchases, but that game can be manipulative. However I think that monetization through cooperative groups can be done fairly. For example, in a game where people formed clans you could allow a member to buy items that, once bought, gave a copy of that item to everyone else in their clan. I'm thinking of something like clothing with a custom emblem that the player can design, that sort of thing. This would allow players to be generous towards their friends, and pay real money to confer superficial status towards themselves within their clan and their clan within the wider game world.

Another interesting new way to monetize skill games would be to monetize through imbalances. Extra credits had a discussion about interesting gaming imbalances, but the general idea would be to regularly slightly change the balance of the game, so players have to change their meta-strategy (i.e. loadout/character/class choices) to keep up. This can then be used for monetization, so players have to keep up with the latest purchase in order to compete with everyone else. So, for example, you might start selling a new frost wand that's a little overpowered, and so everyone wants to play as a wizard. But wizards are slow moving, so a few weeks later you introduce an elf bow that blocks frost. Elves are quick moving, so they're really good against frost wizards. Now everyone wants to be an elf. So a week later you introduce a special move you can buy for trolls that has an area effect (making it easy to catch fast moving elves) that is especially powerful against un-armoured foes (very bad for elves in your game). Now lots of wizard+troll teams are popular, so you introduce a new sword for barbarians etc. etc.

I honestly don't know if this latter methodology is manipulative or not. On the one hand, it means that the game is kept fresh and, because the required skills change over time, it prevents more experienced players from becoming utterly unbeatable by new players. On the other hand, it does seem a bit like a cheap way to squeeze regular payments out of everyone. Maybe a non-manipulative way to do this would be to allow players to ditch their character and play as a new one (with the right class and items) for free, but charge them for having multiple characters. That way you could buy one of each class and then have complete freedom in the game, or you could not pay anything at all, and all you lose are the superficial ways you have customised your character.