So your refereeing a Thief/etc and he hits 9th level. Unlike Fighter or Magic Users they don't build strongholds but instead buy a tower or house and his fame attracts followers. In most campaign the thief is not going to be existing in a vacuum. Likely or not there will be one or more other "name" levels around that PC is going to be horning into their action.
I recommend rolling up a 9th to 12th level thief and using Appendix P in the Dungeon Master Guide (pages 225-227) to equip him. There are also useful charts on page 194, Remember that he part of hierarchy and that he survived X years and Y adventures to get to that position. If he been established for any amount of time there going to be a little society of thieves with a hierarchy.
What I would do is create this background and introduce the players to through the normal course of the campaign. Realize that now the character is "name" level he is developing a reputation and people will naturally flock to him. That what the stronghold rule in AD&D about. It isn't about that you can't establish anything before 9th level but rather when you do reach 9th level your reputation is known.
Of course with this comes new problems. Ones that his adventuring career may not prepare him for. And it will draw the attention of other like him (which you already noted). The game becomes more about the society and culture in which all of them exist. Society and culture will form the battlefield on which these two fight. The focus will not the high level view of Kings and Princes but the underbelly of society.
For two concrete (and free) example I posted two writeup from my long running campaign. The Brotherhood of the Lion and the Beggar's Guild. The Brotherhood is a resistance group several generations after the conquest of City-State. The Beggars are dishonored descendants of the conquerors.
I also recommend tracking down any of the old Thieves Guild Supplement from Gamelords. What particularly valuable are the dozens of scenarios for thieves they include in each book.