A big appeal for Pokemon is the fact you can customise your teams, from a diverse to even a monotype. One thing I really like though is when there is a theme to a character's team in the game.
In particular are the two Water Gym leaders in Unova, Cress and Marlon. How did they differentiate the two gym leader teams of the same type in the Pokemon World Tournament? They gave Cress a general emphasis on freshwater-based Pokemon (Simipour, Golduck, Floatzel, Slowking), while Marlon had large, a more generally sea-based Pokemon (i.e. Wailord, Jellicent, Starmie, Mantine). Those teams also go really well with their personalities with Cress being more classy while Marlon is a big active man of the sea.
Cilan in particular had an interesting theme which was a mix of light, cotton-based, friendly fairy-like Pokemon (Jumpluff, Whimsicott, Lilligant) and Pokemon with thorns in their bodies (Ferrothorn, Maractus, Simisage's tail). I mean sure they wanted more of a Gen 5 team emphasis but it's really interesting if you take that as Cilan being defensive and introverted since he comes across as very timid in the game.
Even the twins Tate and Liza uses a theme of counterparts in their World Tournament things such as Solrock/Lunatone, Gardevoir/Gallade and Gothitelle/Reunicleus. They even have usually levitating Pokemon like Claydol, Bronzong and Chimecho as a nod to how Levitate as one of the more popular abilities in Doubles.
Lastly, any Fire, Water or Grass gym leader will have at least one team that has a starter in the region the Leader was in. For example, Erika with Venusaur and Cress with Samurott (which sort of breaks his usually freshwater theme since I see otters as sea-based). Also, since Sinnoh didn't have a Fire Gym leader (mostly because Fire types were rare in that region), instead Maylene gets an Infernape because of the secondary type!
Oh and N with weather teams in BW2. Nice way to incorporate the seasons mechanic and the power of weather in Gen 5.