They should just make the spawn requirements stricter (for some) and make the spawn rates spammable.

This will ensure players have to put effort in to spawn it, but they will know it will spawn eventually. This will also remove the "has it been killed recently" dilemma of scouring a map for 30 minutes for no clue if it's ready or not. Yes we have Ariyala's, but only the organised linkshells use this. If a random level 70 player found an ARR/HW A rank and killed it alone, likelihood they will report it is low.

Some examples:

Southern Thanalan: Emerge victorious from all fates in a time period. This is an example of a good requirement to spawn something. Not one I'd change as there is dedication to this one
Mor Dhona: Do maps for a chance to spawn, alternatively every 20th (or any other number) map in the area spawns the S rank
Northern Thanalan: Kill earth sprites for a chance to spawn, alternatively every 250th kill spawns it
Western Thanalan (I think): Fish Glimmerscales for a chance to spawn, alternatively every 10th Glimmerscale spawns it

and so on.