Suggestion: Add event npcs that will never reappear, and only some accounts can see

https://oldschool.runescape.wiki/w/2013_Christmas_event (Jack, Jeff, and Jill were visible to those who completed the event.)

Others include:
https://oldschool.runescape.wiki/w/2013_Halloween_event

https://oldschool.runescape.wiki/w/2014_April_Fools

https://oldschool.runescape.wiki/w/2014_Easter_event

https://oldschool.runescape.wiki/w/2016_Birthday_event

https://oldschool.runescape.wiki/w/2017_Halloween_event

https://oldschool.runescape.wiki/w/2018_Easter_event

https://oldschool.runescape.wiki/w/2019_Easter_event

https://oldschool.runescape.wiki/w/2020_Easter_event

https://oldschool.runescape.wiki/w/2020_Halloween_event

https://oldschool.runescape.wiki/w/2021_Birthday_event

https://oldschool.runescape.wiki/w/2021_Easter_event

https://oldschool.runescape.wiki/w/2021_Halloween_event

https://oldschool.runescape.wiki/w/2022_Birthday_event

There may be more that I haven't thought of or looked for but you get the idea.