I would LOVE to see Jefferson again. Just dunno if it’s likely right now, considering every time K&H are asked about him coming back, they talk about Sebastian being a busy guy and scheduling being tricky, but they’ll always have the door open to him to come back if possible. Unless they’re being especially sneaky, it doesn’t sound like there are immediate plans for more Jefferson.
Jefferson couldn’t create a magical hat all on his own, that’s why he forced Emma into making one for him. That hat wasn’t magical when she made it, but it’s probably still lying around somewhere and may be workable now, with magic being back. Who knows if they have any plans to bring that hat back into play at any point. It would come in handy as a plot device if they wanted to bring Jefferson back at any point, though.