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. 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.