I've always been a fan of the stories told in cards.
Tiaane would be an amazing card to see again, just cause of his story in the TV show.
Same with the Assimilated Creatures, especially with Nunk'Worn. Because his entire tribe was wiped out and he's one of the last M'arrillians outside of the Doors now. I could honestly see the Underworlders, Mipedians, and Overworlders demanding his death from the Danians. Combine that with his in-game ability to revive himself, I could see something interesting come from it.
Asides from that Mommark's Creations and Mommark himself would be nice to see again. Maybe designed to create Creatures out of the Creatures in your Discard Pile like: 'Remove target creature in your Discard Pile: Target Creature you control becomes a copy of the removed Creature.' Like Iparu, but it copies Creatures from the Discard Pile.
Although I do agree with your idea of returning Borth-Majar. I could see some interesting alternatives. I could also things like Double Creatures (Single Creatures that count as 2) becoming a possible new card type. Cards like Aivenna and Nivenna and similar things. Two Creatures as one could definitely lead interesting twists.