I've been creating fanmon for fun and working on a fan region since 2014, and now I'm thinking of actually developing it into a full on game called Pokemon Amber.
Without giving too much of the plot away, it'd play like HGSS, and you would be a young trainer living in the Limone region, a region known for its rich history, ancient ruins, and an enormous tree in your hometown containing three giant legendary bugs, trapped in amber. You pick your starter from three fossils that were recently discovered and revived, the fire type Amorn, the water type Aquas, or the grass type Sproutops, and begin your Pokemon journey, unraveling the history of the region and stopping the antics of a team of evil mad scientists called Team Vial along the way. So far I have 67 new fanmon in total, with 3 stage starter lines, 3 legendary Pokemon, and 9 new regional forms, and I'm planning to make more. I also have a whole plethora of interesting characters, 8 gym leaders, a rival, a villain, an elite four, and a champion.
I'm not looking for a hugely successful game, just something people will want to play and enjoy playing. Any advice or suggestions for things I should do? I've never developed a game before but I'm willing to learn, and I'm planning on using Pokemon Essentials to make it.
Non-negotiable: note down somewhere all the credits of all the ressources you are using.
This includes: any script, any tileset, any sprite, any sound that you didn't make yourself.
I keep everything in a Word file that I update everytime I finish doing something. It helps setting boundaries to the fangame. You don't need to make fully-detailed, professionnal specifications, nor should you stricly stick to what you've written on day 1, but you should at least lay down the main ideas. This way, you can also track your progress, as you will have a well-defined goal.
About making a successful game... Success is 80% chance, and 20% how you communicate about it.
But you can check the Game Showcase forum, or the recuitment forum to see how other people do.