Progressive decentralization (part 1)
If you're adding blockchain capabilities into existing games, it would be difficult to flip the switch to full decentralization overnight. Typically, you'll want to progressively add decentralization of different parts of the game. You might even stop at a certain point depending on your goals, the community, and the type of game.
When you're just getting started down this path, the easiest place to start is...
Bridging in-game currency to crypto
You can keep your entire game exactly the way it is today and still qualify as play-to-earn. The minimum bar here is to bridge one currency (gold/gems/etc.) from within your game to a cryptocurrency. There are a lot of considerations on designing this approach, so if you're considering this as your first step, check out Nat Eliason's article on One vs. Two Tokens for games.
The benefit is that now your players can "cash out" or "buy in" without requiring your game studio to take on most of the regulatory burden of being a money transmitter (probably still a good idea to talk to a lawyer). Beyond this, your blockchain integration is entirely opt-in, so players who don't care or aren't interested won't notice its existence. Additionally, you can build this on pretty much any blockchain because there will be relatively few transactions involved (compared with some of the later options), so gas fees aren't much of a concern.
The main problem with this approach is that your game servers are a centralized point of failure. The security and fraud prevention of blockchain doesn't extend to your studio's player inventory database. If anyone hacks your game, exploits a bug, or directly manipulates data on your servers, they'll be able to turn that into real money on blockchain.
So, the next step is...
Moving currency and/or items on-chain
By tracking player inventory on blockchain, you remove a lot of the potential failure points. This requires creating tokens to represent all the different game assets a player can acquire, and then issuing these tokens as rewards via smart contracts. Once you get to this point, having a fee-less blockchain like Koinos starts to make a lot more sense. You're likely to incur massive costs if your game is popular and backed by a chain with gas fees (unless you have players pay the fees directly, but most of the time that would harm the game experience).
When you move player inventories on-chain, you open up a lot more options for your game. You can integrate with a DEX smart contract so players can swap gold for gems or dollars without ever leaving the game. You can integrate with an NFT marketplace protocol and show an in-game item auction house. All without building any server-side logic or hosting your own infrastructure.
There's nuance within this level of decentralization that I'll get into tomorrow.
P.S. in case you missed it, the Koinos governance smart contract is complete, so there's only one thing left to develop before mainnet: Proof-of-Burn consensus.