Who said there are no Contests in Unova? We have never seen a Contest in Johto, but we know they exist! Writers can say that we not see Contests in Unova because none of Ash's friends participate them. They can bring Dawn to participate in a Fantina/Juan Cup. And Brock can go back as well, just say that he is interested in studying the Unova Pokémon, especially Audino.
Agreed,plot dictates what will happen and if writers decide to make them relevant to BW storyline,distance certainly wont stop Brock or Dawn from traveling over there.
Speaking of contests possibly existing in Unova,thats interesting argument you brought in.We didnt seen contests in Johto and Kanto as well during original series but that doesnt mean they didnt existed as it was later showed with writers at that time not having reason to show them since there wasnt coordinator in main group.
Though idea of Dawn coming over there to promote idea of contests giving us some update about her journey in Hoenn would be nice way to bring her too.
Speaking of Brock having him travel to Unova with purpose of gaining some knowledge and met some famous doctor which would help him before entering test in attempt of becoming doctor would definitely be one of good reasons to have him show up there.
Why do people even suggest things like this? You know good and well that Misty probably won't appear in the anime ever again, until a possible "series finale" type deal.
So wait its perfectly fine wanting Dawn,Brock,May or any other character back but its unreasonable to expect same thing for Misty?
And i fail to see how she wont return ever again.Misty was first and original girl being very popular and liked by many along with original trio making show popular and worldwide recognized achieving iconic status.If minor characters like Gary,Jessibelle,Jasmin etc managed to return there is no reason why she couldnt as well.
Her story was left unfinished and she definitely has potential to develop more so lack of reasons isnt really present either.
I could see her coming in Unova:-for some water tournament to advance her goal of water master and display skills as trainer among othrs learning jnw things from it
-help Iris about possible gym issues contributing to anime plot since she is gym leader too(similar how May did by helping Dawn to get over her confidence issues).
-catch new water types which arent possible to obtain in Kanto(like its case with Unova pokemon) and gain new knowledge.
Etc,etc.