Just had an idea. I used to really enjoy the bug-catching contest in Gold/Silver and thought something like that would be great for Topaz. Instead of a bug-catching contest, how about a fishing contest. I can see it being located on one of the long routes in Kirant, helping to break it up. Its an excuse to give away exclusive items or TMs, or even an HM. The pokemon I can see being available might be:

Remoraid (Very Common)
Trillogill (Common)
Corphish (Common)
Chinchou (Common)
Octillery (Uncommon)
Crawdaunt (Uncommon)
Lanturn (Rare)
Reefrain (Very Rare)

However I'm not sure how easy this might be to program