Having a solid way, even though Dnite can carry Tpunch and Elec-Z, to hit Water-types could help. Tapu Koko, for example, is a good option to add to that (already amazing) core. Then you can play around it and form another core, for example: Koko / Mega CharY / Landorus.Been running the somewhat viable Hippowdon, Mega Luke, and Dragonite core to a mixed success, but the one problem I have is I am not sure what teammates usually run with them. Has anyone else used this core and can give me a potential answer?
EDIT: Also what other viable Mega Luke teams have gone around gen 7 BSS?
Or you can just play the 3+3 way without adding any other tool to that core. Having the Hippo-Dnite-Luke core and another indipendent core that does well versus when the other one doesn't.
To answer to your second question, if you don't want to use Luke with Hippo and Dnite, Zapdos, Tapu Lele, Tapu Fini, Primarina, Rotom-W/H, Mimikyu, Gyarados, and similar mons are all good partners with whom you can build a team.