Batching allows us to combine multiple things under one word mechanically in an easy to grok way, if we can find a word that describes said things in a way that makes it easy to understand. Fortunately there exists a unique word with no prior mechanical baggage that can be used to describe both snakes and naga (and serpents! But I'll get to that): ophidian.
An ophidian creature is one that is of serpentine (or snakelike if you will) nature, which makes it perfect to use to refer to both Snakes and Naga, as they are both very serpentine. It feels a little bad only using the word for two creatures, but looking over the various creatures of MTG, there is another creature type, for which ophidian can be used as a descriptor, the almighty Serpent. That gives us a batch of three creatures: Naga, Serpent and Snake.
Which leaves us with the rules text when something refers to something ophidian: (Naga, Serpents, and Snakes are ophidian.)
Simple, and to the point.
Onwards to the examples!
Rana, Serpent's Kiss
Legendary Creature - Naga Assassin (Mythic)
Deathtouch, Vigilance, Menace
At the end of each turn, if a creature was put into an opponents graveyard that turn from the battlefield, put a +1/+1 counter on each ophidian creature you control. (Naga, Serpents, and Snakes are ophidian.)
3/3
The Ophidian commander.
Darkhollow Shaman
Creature - Naga Shaman (Uncommon)
Menace
Other Ophidian creatures you control have menace. (Naga, Serpents, and Snakes are ophidian.)
2/1
An uncommon Ophidian lord.
Immobilizing Venom
Enchantment - Aura (Common)
Enchant creature
When Immobilizing Venom enters the battlefield, tap the enchanted creature.
As long as you control an ophidian creature, the enchanted creature does untap during its controller's untap step. (Naga, Serpents, and Snakes are ophidian.)
Shadowscale Regent
Creature - Naga Warrior (Rare)
Hexproof
Other ophidian creatures you control get +2/+1. (Naga, Serpents, and Snakes are ophidian.)
2/3
A Rare example of an ophidian Lord.
Anyway, you get the general idea.
As always comments, critiques, and constructive criticism is always welcome, as are additions.
Edit: Fixed capitalization issue.