There is fanart of the smash cast just hanging out and living together. Similar to the Creepypasta Mansion. In the art I've seen they mostly share bedrooms together. But I'd like it more if everyone to had their own rooms. Because then you can stuff these rooms with references.
The overall color scheme is sunflower based : Green, yellow, orange, brown.Having sunflowers as houseplants is impossible so as substitute there are diffrent houseplants on the window and a poster of sunflowers next to his bed.
There is a big mirror to recreate the hairbrushing(mom flashback) scene from the game. There are pictures of his family right behind him so he can look at them through the mirror. He also has pictures of his friends on the mirror, Lucas is the only boy who has something like this. Having a mirror with pictures on it is pretty girly, but he lost people before so he makes mementos of anyone he likes just in case he loses them too. (Having your picture on his mirror might be a death flag.)

The snake he ripped off from Duster has a terrarium and a poster of DCMC (Duster's band) is right next to it. Giving mice or other animals to the snake is too cruel for Lucas, since he can talk to animals. Luckily the Pokemon world developed the perfect kind of substitute food for carnivores so Lucas doesn't have to kill mice.
He has a cassette player, with a DCMC tape, as an alarm clock. The world he's from still has brick cellphones, so him having cassettes makes sense. He has a GBA because that's where his game was released on. (everyone should have the console or handheld their first game was released on)
His stick weapon is right next to the rope snake. And he has a flower vase next to the picture of his mom.
The room is very tidy I don't think Lucas would have a messy room.

Other rooms can be messier depending on the characters personality and how much stuff they own.