In the future that is Neo Gotham, Batman isn't the only one who has left a legacy. The Joker too has people who take up his mantle. The Jokerz aren't just singluar people though, they're a gang, or several. In fact, in the Batman Beyond comics they're noted to be in such numbers that they filled an entire rally nearly 10,000 strong.
Usually, they just like to cause havoc, mess with people, but occassionally, there are those who have bigger visions, not unlike their inspiration. Either way, they're a big pain for Batman, same then as now.