The only possibilities I can think of are [name]Sidony[/name], [name]Sidney[/name], [name]Saraid[/name], [name]Sarai[/name], and Duvessa (which is a huge stretch, as the d and the s are in the opposite order as they appear in [name]Sadie[/name] - then again, [name]Sarah[/name] doesn’t have a d sound at all!). Know of any longer/less nicknamey names that include the s and d sounds?
I really like it as a nickname for [name]Isadora[/name], though I like it on its own, too.
I think [name]Sadie[/name] is a name on it’s own, there is no long version really…
I didn’t think of [name]Isadora[/name], though it seems so obvious now! Thanks.
[name]Sadira[/name] is another option, as well as [name]Sadiya[/name] and [name]Sedona[/name]. I do think [name]Sadie[/name] could stand on its own, though!
Although traditionally [name]Sadie[/name] is a nickname for [name]Sarah[/name], I agree with the other pp’s who’ve said that it can stand on its own. I know two little girls who are just named [name]Sadie[/name] and they seem fine with it.
[name]True[/name]. But for name nerd reasons, I’m just curious as to what could work as a long form. 
The only one I can think of that hasn’t been mentioned is [name]Mercedes[/name] nn [name]Sadie[/name]. I actually really like [name]Isadora[/name] nn [name]Sadie[/name], though!
I’ve heard of [name]Seraphina[/name] being the long form for a little [name]Sadie[/name] but I like [name]Isadora[/name] BN [name]Sadie[/name] a lot better!