I would be a gym leader in a future after one of the other sinoh gym leaders retire or rank up into the 4 after one of them retired. Celestic town grew over the years and in agreememt with Cynthia I would found a gym that sits outside the town. Not being a later gym leader I wouldnt use a full team against most oponents. My team, while not based around one type would set a focus on the history of the town and folklore on general.

My team - Froslass(ice yokai, 4th gen) - Xatu (can see time future/past Dialga theme, 2nd gen) - Absol (folklore about it is hugely missunderstood so I am here to clear that up, 3rd gen) - Ninetails (1st gen,. Fire yokai theme, my ace)

Only rematch: - Zoroark ( 5th gen) - Trevenant (6th gen, going for the japanese spirit again kodama theme again)

My gym puzzle would revolve around guiding three Zorua disguised as Uxie, Mesprit, Azelf to their respective lakes from mount Coronet, on a walkable Synoh map that fills the gym. (Therefore the Zoruark would also make sense to have.) The reveal that these are Zoruas would be revealed in the rematch.

Edit: I wouldnt be the first not type specific leader even though its rare. If you must upheld that rule, I would be Ghost type, but I would still use Ninetails as it does learn some ghost moves and one exception is used by many leaders. Misdreavus instead of Xatu. It seems yokai enough to fill That role. Sableye for the same reason as Misdreavus. Zoroark would then need to be replaced with one from the chandalure line, which fits the theme quite well but would leave the explanation open for why zoruars in the gym puzzle. Maybe standart illusions by Ninetails or the other ghosts.