Make up some requirements to be able to create a guild (maybe there is a guilds-guild where you need to register). Requirements could be:
A fee to register your guild. This could be very expensive. Maybe they can find an investor but need to do a favour for them first.
You need a location inside Waterdeep. Also expensive, maybe they can get one via a quest. Or if they have Trollskull Manor, they could make some changes to the building to make it suitable as a guild house.
You need at least 15 members. They need to try to recruit people.
…
Also, maybe some other guilds are not happy with this (they think there are already too many guilds, or the guild is too similar to their guild) and they try to sabotage the players.
deleted_by_author
Waterdeep Dragon Heist: Establishing a new guild
a player of mine would like to establish a new guild....