Bloodbraid Elf, Kolaghan's Command, and arguably Kroxa are all two-for-ones, but they're the equivalent of a burst of nitrous into the engine rather than a fuel refill
Partnering with Vilis is the extraordinarily powerful Dark Confidant, even if it is a bit of a non-bo with Vilis
If the copied creature is a token, Kimahri copies the original characteristics of that token as stated by the effect that created the token, with the listed exceptions
Kaalia Dragon Enablers While not dragons themselves, Kaalia of the Vast and Kaalia, Zenith Seeker are incredible for any dragon strategy
Combined almost exclusively with Ensnaring Bridge, Grafted Skullcap provides a self-Howling Mine effect with a drawback lose your hand at the end of each turn
It is always huge and offers significant damage output or potentially even some life gain