I'm not too sure how that would have worked. I love Oz and Cordy, but I don't think they would have worked well with the later Buffy seasons. In the case of Cordy, it would have been interesting to see her dynamic with the group evolve, but there's a few issues with her staying. One, she's one of the characters that literally has no reasons to stay. Aside from fighting against evil, give me one reason as to why she should stick to Sunnydale, cause personally I can't really find any reasons. Two, she fitted much better on Angel, it suited her personality better in my opinion. Cordy needed something to push her towards evolving as a person, becoming a better version of herself. Honestly I can't see that happening around the Scoobies. Cordy needed a family, and she didn't fit the Buffy and co like she did with Angel and co. Surely the dynamic with the scoobies would have been interesting, I won't deny that, but as a character it was best for her to move on.
As for Oz, they could have done something interesting with him, but he suffers the same treatment as Cordy I believe. He plateaud a bit as a character, in order for him to grow he would also need to move on from Willow. New Moon Rising shows that Willow stalls him as a character. Oz would have done great on Angel, it's a shame there was no s6. But honestly, I would not have been intersted in seeing 6 seasons of Woz. They were great as highschool sweethearts, but that stops there for me. And I have no interest in seeing a love triangle between Oz, Tara and Willow.
The problem is that both Cordy and Oz played a bit of the gf/bf role. Those characters tend to grow stale if they remain in that role. I think that's partially why many don't like the Spike focus in s7. When a character is meant to be a gf/bf material, their storyline can become redundant. That means they need to bring the character out of that gf/bf role, as they did with Angel on his own show, but that would be a tricky thing to do with Cordy and Oz. Might have worked with Cordy because out of the highschool years while she was hanging out with the gang, most of the time she was actually not dating Xander. But Oz would always be "Willow's bf", there's no way around that one.
Anya and Tara worked well because both were on the show for only a few seasons, and by then I'm sure many will even agree that Anya was passed her prime in s7. To me, BtVS will always be about Buffy/Xander/Willow/Giles, that's the four we started with as a core group. Adding anyone else just feels like adding an outsider to the group. Meaning they can never truly be part of that core 4, which limits what they can do with the character. That is even worst when we're adding a new character solemnly as a bf/gf. I don't know if that makes sense, but to my Cordy fitted better on Angel because she's part of the core on that show. She wasn't made to be a secondary/gf character, but to be part of the main focus. Even though she was on Buffy from s1, she wasn't part of the core. AtS works as a assemble cast because there is mostly no romantic relationships between the parties. Buffy was always B/X/W/G + the significant others. That means there is a necessity to recycle the bf and gf to make sure that B/X/W/G keep evolving themselves.
I'm not sure if that rant even makes sense, but I'm trying to say, I don't think there was a place for Cordy and Oz in Sunnydale past s4. But I think there was room for both characters on AtS. It would have been a different story if the characters would have been introduced as something else than just a sidekick/SO (like Dawn or Faith, who were introduced as important parts of the story), but they way Cordy and Oz became part of the group is somewhat limiting.
