There have been promises of some legit "surprises" from the series in terms of cameos/connections/etc. There's no way the first ever SW series is going to be *that* removed from everything fans know and love about the franchise. While I can't see Harrison Ford participating or lending his likeness to any kind of de-aging process, I'd bet just about anything that a de-aged Hamill/Luke shows up at some point, in a very limited capacity. And seeing as the First Order is mentioned in Favreau's announcement (even if it's to say they don't exist yet), I bet we get some kind of First Order rumblings as well - maybe brief glimpses or hints of them first coming together, or even hints of Snoke, etc - again, even if only referenced and not outright shown.
Other potential connections/cameos...
- A de-aged/CGI Liea appearance (though I doubt they go there)
- A younger Lor San Tekka
- Poe Dameron's parents
- An old Qi'ra
- And maybe even some kind of brief wink to Rey's "nobody" parents
Trust me, I'd love for this series to stand on its own as much as possible, and I'm the last person who wants this galaxy feeling as small as it does, with everyone popping up in each other's stories, but the reality is that Disney/Lucasfilm will want as many wink-wink connections/cameos as they can get, but I trust Favreau & co to hopefully deliver them as elegantly, organically, and smartly as possible.