Fortify health, magicka, stamina. If they bring chameleon, shield, feather and others back it would still be useful.
Indeed, there are quite a few effects already in past games that can still be useful without attributes, both the ones mentioned and fortify skill enchantments, there could also be new effects available for enchanting as well. And even without attributes in the usual sense, that doesn't mean we couldn't have enchantments that replicate the function of fortify attribute ones. Maybe some items could have an enchantment that increases melee damage and carrying weight, thus essentially being fortify strength, and maybe some items increase your movement speed, just because there isn't a speed attribute doesn't mean there can't be factors that determine how fast you move. Removing attributes certainly removes some options for enchanting, but there are still quite a few others, Bethesda just has to ensure they're useful.