I highly recommend signing-up for H&C TV or even wehorse.com for some virtual lessons on long-lining and double-lungeing. A lot of the ridden dressage content is surprisingly applicable too, actually (although of course you have no seat or legs to work with).
I think the winter can be a great opportunity to go back to basics and work on those formative building blocks of rhythm, suppleness, and contact… without anyone needing to risk their neck being in the saddle on a fresh horse!