I think a majority of the set shouldn't encourage people to go for every card type at once. A couple "domain"-like cards (like from the mastery proposal) at higher rarities is fine, but I don't think it's interesting at all as a major theme--players play with a mix of card types in
every environment, so in the end the "card types matter" theme ends up just being "make sure you have a couple artifacts and enchantments." I think the far more interesting and unique direction to take is allowing/encouraging players to focus on one or two card types.
Think about Lorwyn--it doesn't achieve the tribal (i.e. "creature type matters") theme by encouraging people to use a mix of every kind of creature, it does it by dividing the creature types into a few overlapping subsets with unique identities and playstyles. It also doesn't do it by being solely a "treefolk matter" set--obviously it's different when our factions aren't just different kinds of creatures, but I think a pluralistic theme is perfectly viable.
Also keep in mind that a deck doesn't need to consist entirely of a single card type for that card type to "matter," and that not being focused on creatures doesn't mean creatures can't make an appearance. There are artifact creatures, (probably) enchantment creatures, tokens, and permanents that turn into creatures even if they aren't generally of that type. There are supporting creatures like Frostblight Scavenger and
many more to come. There can be creatures that are meant for combat, but still depend on other card types, like
Kiln Fiend or whatever. Or someone could draft a strong "instants matter" deck with removal and counterspells and recursion and just run a couple unrelated creatures for win conditions. I count all of those as victories for the set theme.
Anyway, from my perspective, we're not looking for a keyword that tells everyone to play every card type. And making a separate keyword for each of five or six card types is probably ridiculous. I think our best bet is something parametric like
kinship that could go with a number of different card types.