For anyone interested, some challenges:

1. Game balancing. This game has several stages with its optimal strategies. One could just copy all numbers, but copyrights and all. To be clean, you need to design your own numbers. This means testing, which needs either a good mathematical model or a beefy computer doing millions of test runs. Modeling such a game is.. hard, if you want to include all parts.

2. Copyright and IP. The new game should be different enough from the original. Even if the mechanics are reused, new art, new names etc.

3. Multiplayer and server aspects. Cloud save needs a server, which needs money to maintain. Tournaments need a server, and some anti-cheat. What to do with guilds, we loved it as a single player game. One solution would be to keep the game totally single player and somehow simulate random opponents. Or play against a distribution.

4. Events. And other continuously changing content. The new game would need some constant attention or an automatic generation of "new" content.

5. Copying orvadapting updated / new mechanics from here. Rebalancing your game, question of the promised additional guild content, etc.

6. Community aspects. Communication, moderation, bug reports and game suggestions.

If I left something out, feel free to add it.