They can spawn in the world near players at random intervals, with a minimum of three in-game days in between each gateway.

Other ideas to compliment this:

• They only spawn at night
• No rewards for naturally spawned gateways outside of the drops from enemies you kill.
• They can only spawn in a spot where the sky is visible from above, to prevent getting ambushed in your base
• This whole feature can be toggled on and off, as it may not be for everyone

I still have not put any time into adding this natively, but someone else has done it with a datapack https://www.planetminecraft.com/data-pack/gatewaybynight/

Making this a native feature or at least adding the link for the datapack in the mod page would be great. Gateways that spawn randomly and release monsters is a common trope in most korean novels and manwha, and there are also a lot of anime with similar plots. So if people knew that you can make portals spawn naturally, some anime or manwha based modpacks that use this mod might appear.

No interest in doing this for the time being.