I know, some of you think I'm insane and that I'm just a Pokemon purist, but hear me out. Like others, I grew tired of the monotype gym system which had a challenge only on the first time you played. When it was revealed that Alola would forgo gyms for more of an island challenge type thing, fans were curious to see how it went.
And, from what I've seen, reception has been mixed to positive. Some people gave Sun and Moon credit for wanting to be different, but other fans were fine with gyms -- they just wanted a rehaul on the system.
With that said, I do think that if gyms are to come back -- which I wouldn't totally be against -- then they need to be different and have true 'themes' if they're to properly last. Generation III experimented with the gym system when they had Tate and Liza's double battle gym in Mossdeep, while Blue's Viridian City Gym in Heart Gold and Soul Silver lacked a specific type.
A pure ground or fire gym is useless at this point, but a gym that focuses solely on Trick Room users would be both fun and a challenge. What about a gym with an emphasis on combo moves (i.e. Rain Dance + Thunder) or even a return of the double battle gym -- but with different types?
Oh, and we actually need a dark type gym. How have we never had one when the type was introduced in Generation II?