Eh, I don't really see the need for a pactio with Sakura. It just seems kinda like it would clash with Clow's style, which Sakura's is based on.
Besides, in practice, they operate kinda like the Eastern system, with familiars and such protecting the master. The Western bit, is probably the Cards themselves. Part Pactio, part magical creation, part artifact.
... Wow, that meshes pretty well.
Going to Mahora can be either something suggested by Eriol, or just transfering there because of her father's job. (Mahora is part university, afterall.) Tomoyo comes along, because she's like that. Li goes along, because Sakura is his entire reason to be in Japan. Anyway, given the timeline, it sorta makes sense. Sakura bound and transformed all the Cards before she went to middleschool, afterall. This would be about the time that Li comes back from Hong Kong.
I can also see Negi spazzing out about how the LEGENDARY CLOW REED's tools of the trade, his AMAZING BOOK and CARDS ... are now owned by one of the students in his class. Given the fact that he's antique freak, I can just see him going BUW-WAH?!
Also, this gives Sakura (later), a chance to expand her talents... into combat. Because, it'd be totaly awesome. Well, Sakura probably wouldn't like it one bit, but whatever.
And I wonder how Eriol and Negi would get along...