Your topic title immediately made me want to post a joke about the fun of using RPG in first person shooters (Rocket-Propelled Grenade)... ;-)
I like RPGs (the Role Playing type) for a wide variety of reasons: the engaging story, the sense of growth when leveling up, the sense agency when when choosing how to allocate stat points and choosing equipment. I also like the exploration, player community, the puzzles, ...
Action RPG add a sense of control and mastery because of the execution challenge it presents.
Tactical RPG makes you feel like a strategy mastermind.
Some really niche RPG come with unexpected story and protagonist. Three the Hard Way is a RPG I played a long time ago where the protagonist starts as an anti-hero, the growth is quite original. I like the unexpected story developments. And as many hobby RPG, you can reach the developer if you want.
I also like the relax atmosphere of Open Source MMORPG like The Mana World that has a very friendly community and gives you the opportunity to contribute, raising your IRL development skills.
Table Top RPG really push your imagination both as a player and even more as a Game Master.
Some classic RPGs like Might and Magic 6 & 7 have a large open world you explore with a 4 players party you build. Choosing the party composition that best fits your play-style is very fun. The world also change depending on your actions good or bad.
Baldur's Gate also comes to mind as a great RPG. Here you choose your companions as the game story progresses. It offers a lot of replay value because you can branch into very diverse party compositions and story developments.
You can also think about Undertale for its player-choice driven consequences (some times heavy).
Apart for the initial failed joke, you see that they are a lot, really a lot, of styles of RPGs and a lot of reasons to like them. BUT ! It is impossible, and not wise, to put all of them in a single game. You should focus on the aspects you're most comfortable with.
Do you like to write deep character stories and wold developments?
Do you want to create a challenging tactical combat system?
Do you want to develop a wide array of party customization?
Do you want to build a online community game?
Do you want your project to be open source so other developers can join you?
Do you like survival elements and resource management (often found in post-apocalyptic)?
What theme do you like? Medieval fantasy? Futuristic? Western? Comedy?