Depends largely on why you want to train in another art. Is it to cover for perceived flaws/weaknesses in Aikido? Is it to help your Aikido? Tai Chi and sword arts can supplement Aikido training. If it's the first reason, where do you think Aikido is lacking? Personally, I'm not a huge fan of trying to study more than one art at a time. I think you can end up knowing a little about a lot but not much of anything.
BTW, Judo won't help your Aikido hip throws very much. Apples and oranges.