I like it, but if you use it on a child you need to be okay with
Cassie because that's probably what she'd be called. That or
Cass, or maybe
Ana.
Elise doesn't work because it's too many S's, and
Harper is too boyish and clashes with the style of Cassiana.
I like all the other middle names you listed... Although
Eve is a little awkward because Cassiana ends with a vowel so putting
Eve after it forces an awkward glottal stop. It looks nice but it's harder to say than the others.
In general I'd prefer the sound of Cassiana with a one-syllable middle name but those are hard to think of.
If I had to name my kid from your list I'd pick Cassiana
Laurel, or maybe
Violet.
Trying to think of more one-syllable options--
Cassiana
Laure Cassiana
NiamhCassiana
Wren Cassiana
Rue Cassiana
BelleCassiana
VinCassiana
RoseCassiana
Gwen
(thinking of one syllable name is hard...)
Cassiana
Titania Cassiana Lorien
Cassiana
Willow Cassiana
PersephoneCassiana
Primrose