So my idea is about a gen 3 hack called pokemon star! It will have some gen 4+ pokemon, no hm's (why not) and some NEW pokemon! Also, time will have something to do with this rom... and timetraveling too... And, of course, BIDOOF! I'm still in the start learning the basics, but i hope i'll make it! Any cool ideas to add in the game? Also, soon i'll put here some "spoilers" for the game (don't worry, i'll not spoil the whole story) and other thingies. I hope that there will be a lot of ideas from me and other people for this game, so i can make it the best i can!
Sorry for bad English, it's my second language.
Well, since it's called "Pokemon Star", you can add Solgaleo and Lunala, and maybe Minior (you know, it is literally a star).
About the graphics part, you can make it with the graphics that essentials has, but I recommend you to use your own graphics, but you will need 3 things: a Tileset, OverWorld sprites, and Sprites. Tileset is the group of graphics that you'll be using when you make the map of cities, routes, caves, etc. OverWorld sprites are the characters in the world, who can move around, give you things and all that stuff. And, the Sprites are, for example, when you enter in a battle with a Youngster, it will say: "Youngster () wants to fight!" and it will show his sprite.
I really recommend you look up for tutorials on youtube, like the ones of Thundaga, and try them by yourself, with that, you will learn and gain experience.
Also, if you want to add some new mechanics, like following pokemon, you will need to learn some basics of scripting, and follow the tutorials on the web (Please, make sure that your current version is compatible with the script itself)
Good luck! Tell us if you have a problem with something, we'll try to help you!