that would be a good idea, but the question is how would you manage to beat every trainer that came to challenge you? Meaning would you have the time to accept multiple battles? Sure it would be great training for your pokes but some people don't like to just battle in the pkmn league..they like to spread their reach to other places (like contests, battle frontier, trading, etc.)
I'm up for the idea though. It would be even neater if the challenger was say not the computer but someone who owns another black or white version..nothing would be more shameful than losing your title to somebody in Lagos when you live in America.
good idea, but other than that I'd like it if we could battle trainers on city streets too. Like you can already battle some people indoors, but it seems like in the city or town 3/4 of all people are just useless and only there to fill up space. Replace at least 2/4 of these people and now you have many more trainers to fight outside of routes and gyms.