With so many desktops, why don't you contact Canonical? They're experts. It's what they do. You're not saying why you're looking for an alternative to Landscape. You're actually not saying much at all.
If you just want to manage users, storage, etc, then an LDAP+NFS-setup might be appropriate and you have lots of software to manage those services. If you want to upgrade many computers at the same time, then it's possible to do that using ssh and some scripts. But until you provide more information about your needs and why Landscape isn't suitable, it's impossible to provide a detailed answer.