Just wondering what some upcoming themes where for Dragon that are in the planning stage please? This is for the purpose of proposing articles.
My advice would be to send in your ideas, regardless of what the plans are for future themes. For example, when I pitched the idea for the initiate feats and spells, I had no clue they were planning to run a "God-centric" magazine this past April. Sometimes your ideas will just fit into the theme; sometimes, your ideas might give them an idea for a new theme; sometimes, your ideas will stand all by themselves and just be cool regardless of what issue they're in.
Also, I'm willing to wager that any themes that are taking shape already have the majority of their article slots filled. That's only a guess, though.