If you're ok with period dramas :
Jumong (81 episodes / a classic)
Queen Seon Deok (62 episodes / great period drama with gender-bender in quite a lot of episodes)
Emperor of the Sea (51 episodes)
The Kingdom Of The Winds (36 episodes / about Jumong's grandson)
Goddess Of Fire, Jeongi (32 episodes / gender-bender / in your PTW list)
The Bridal Mask (28 episodes / not that much in the past, it's under japanese occupation / in your PTW list)
Faith (24 episodes / in your on-hold list, a lot of special effects so it's not everyone cup of tea :p)
Gu Family Book (24 ep. / fantasy / in your on-hold list)
Hong Gil Dong (24 ep. / sort of a robbin hood)
If not :
King of Baking, Kim Tak Goo (30 episodes / I don't remember but I think a lot of people liked this one)
Goong (24 episodes / what would happen if there was still a royal family in korea)
Dal Ja's Spring (22 episodes / romcom / noona romance)
