As for a theme, may I suggest a "Walk down memory lane"? For instance, Johto, Sinnoh, and the newly introduce Unova, are all fresh in our minds. However, whatever happened to the glory that was Kanto and Hoenn? It all started in Kanto, the games, the pokedex, the series, while Hoenn had hands down the best story line, some of the best pokemon (Gardevoir, Salamence, Flygon, Metagross), and emphasized Pokemon's biggest them; nature. How abot "Use only Hoenn and Kanto pokemon to create a symbol to represent the forgotten Gens". Perhaps not that complex, but I'm sure you catch my drift.