The classic Pokemon gym system has become repetitive and is in need of revitalising, hence Alola's trials and the new requirements for LGPE.

How would you go about shaking things up so that gyms were more interesting and less melancholy?

I think that gyms should specialise in other aspects than just specific types.

For example, a set of 8 gyms could specialise in:
>Double battles
>Tripple battles
>Rotation battles
>Inverse battles
>Weather abuse
>Stat raising/set-up abuse
>Stall team
>Glass cannon team

This would instead test the player with different strategies and situations rather than just steam rolling gyms with a type advantage.