In vanilla, flaming arrows, whether shot from a Flame bow or through lava/fire, can light TNT, ignite campfires, and set entities on fire... BUT NOT NETHER PORTALS!

Nether portals can be lit by flint and steel, lightning, ghast/blaze fireballs, fire charges, and FIRE... BUT NOT FLAME ARROWS!

Well, now the two work together as they logically should.

ANY arrow on fire will be able to ignite any Nether Portal that fire would light. (This datapack doesn't change anything about what is or isn't a valid portal, so if you have a datapack allowing different portal shapes or sizes, this will work with it. The arrow does have to be stuck in obsidian, however - so if your datapack allows wool or concrete nether portals [​is that a thing?], this wouldn't work with it.)

The datapack only triggers when a flaming arrow is stuck into obsidian, so there's still no need to worry about setting your house on fire after you missed that creeper 6 times. And what if your house is made of wool and obsidian? Well, the fire the datapack creates flashes on and off in less than a tick, and isn't enough to ignite surrounding blocks. I tested this by firing dozens of flame arrows into an obsidian line touching a wall of wool (with fire tick on, of course). Even with random tick speed doubled, all the arrows burnt out without igniting the wool.

Also, this datapack works as long as a flaming arrow is stuck in obsidian. Which means if there's a flaming arrow in your portal frame, and your portal is somehow broken, it will reignite itself.