I am the ultimate novice on RMXP. But I have great ideas. I have the idea for a Hoenn sequel. But it happens much later.
Here's the ideas-
Takes place in Hoenn
Play as Unova/Isshu Players
Ash and Red are both in-game.
Hoenn makeover (so your old maps won't work, even on routes)
Generation 5 overworlds
All Gen 1-4 Legends available
No Gen 5 pokemon available.
New Gym Leaders/E4/Champion
Starters are Shinx/Diglett/Poochyena/Natu
Access to Johto/Kanto
9 new Pokemon-
Charxet, Mallachar, Aerochar
Eelurtle, Turteel, Electreel
Leefy, Leefix, Woodleaf
StarterX Catalog to order Pokemon eggs
Team Sky along with Magma/Aqua.
You just moved in from the Unova reigon to Litterloot town in hoenn. Your mom tells you to go unpack your stuff and while your up there some thugs come up and steal a weird mirror. your mom comes upstairs and asks you what was the commotion. You say it was just a box that tipped over out the window. She pulls out a pokeball and sends out a Quilava that uses Recover on the window. Then she asks you to go outside where you find Professor Birch walking by. He notices you and tells you to visit his lab later. You go to the lab and see the thugs from earlier atttacking the Professor. He throws his briefcase toward you and you can chose from some pokemon-
After you chose one of the level 5's you battle the thugs-
Mystery Thug is their trainer class at first.
#1 has a level 2 Cyndaquil
#2 has a level 2 Totodile
#3 has a level 2 Chikorita.
Then they reveal that they are grunts from Team Magma, Aqua, and Sky.
Team sky is a new team who seeks to catch the legendary pokemon rayquaza.
They wear Pilot clothes and are called pilots, not grunts.
The grunts and pilot run, knocking over the briefcase and knocking the pokeballs out.
Then Professor Birch thanks you and gives you a pokedex. You are about to exit but Ash comes and asks Birch for a Pokemon. Birch gives him a choice of Pikachu or a master ball to catch a wild one. He says he already has a Pikachu which is obvoius because it walking with him. Birch gives him a masterball and Ash sees you and tells you to find him outside after he catches a pokemon. Red comes and asks for Birch's pikachu. Birch gives it to him. Then Ash and Red start arguing about who's pikachu is better. They have a battle and Ash beats Red. Red runs outside as to heal his pikachu at home. (by the way, ash and red both moved to hoenn the day you did). Birch picks up the pokeballs on the ground. You find Ash in a patch of grass. (litterloot has some grass in Collision) He runs into a wild charmander and decides he doesn't want it. He tries again but this time runs into a squirtle. He sees you and asks where a good place to find pokemon is. You say you also just moved here so you have no idea. He walks to the first route, I lost track of hoenn route numbers, but it is renamed grass path 1 anyway. here is where your hoenn adventure really starts.
Some stuff to know-
All Generation 1-4 pokemon are found here even though generation 5 pokemon are in your old hometown nuvema. No generation 5 pokemon are available. You can catch all the starter pokemon in litterloot from every generation. All legendaries from Generation 1-4 are available in a place after the league. The trick is once you beat the champion, Birch gives you a mystery orb that summons Mew and Mewtwo. Once you have Mew and Mewtwo, put them in your party and talk to Birch again, who will give you a Mysterious Plate, that summons Lugia and Ho-oh. If you catch them, Raikou, Entei, Suicune, Articuno, Zapdos, and Moltres will be summoned. If you catch them, Birch will come there and tell you to hurry up because 2 Mysterious Pokemon are changing time. The he disappears and repeats the same thing and this time you run out and (the cave is in litterloot but you must beat the league otherwise a Youngster will tell you that it is too dangerous (it has level 70's in it im serious so don't attempt until you have at least one level 80+ and have beaten the league) Dialga and Celebi are rampaging like crazy. You travel back to when you exited the cave. You run and have to fight Dialga. Once you have it, Celebi travels itself back in time. Dialga, who is still in your party, travels back to before Celebi went back and you battle Celebi. Catch it an you get an item that appears from nowhere in your bag called Legendary Pokemon Set1. This is all I have so far.
There is alot of in between story but this is what I have for now. I am the worst on RMXP and I tried to use events but I can't do it. But I have the idea, and that is what counts!
By the way, I'm using Flameguru's Pokemon Essentials so if you can use that great.