You can by: (the more help there is the faster and more often it'll update
Creating Towns/Cities, Routes, building indoors, etc. Preferably Gen4-ized. MOST needed! (custom ones could help for variety)
Overworlds & Trainer sprites. Monster & 'Trainer'; latter Gen5 style.
Item Overworlds, such as more Pokeball types (you know the little item balls) & Pokedexs, and such (Poketch, Xtransceiver, etc).
Spriting Pokemon-style other mons, such as Digimon using various Pokeparts - there's plenty of monsters here; plus back sprites. Prefer Gen4 monster style sprites. HG/SS
Re/De-vamping sprites to HG/SS style such as "GHOST" from original games and Reshiram from Black & White.
Submitting character ideas, be them a pokemon species or otherwise (lab-experimentals be allowed; such as impossible hybrids or say flying type immune to electric & ice or a grass-type using a fire-type move, ask for others)
(use your imagination, can be already canon towns/cities or non-exsistant ones, all works! Giratina's Reverse/Distortion World)
(ask for non-pokemon if you want to do one of them, plenty where they come from)