Isla Charlotte is my current choice with Isla. I also like Isla Beth. Isla Kate is OK and I'm not crazy about Isla Tess.
Isla Elizabeth has too many of the same sounds for me. Isla Summer is pretty but since Isla is a little trendy, personally, I like it with something more classic. Here are some ideas I've played with --
Good luck! I love the name Isla. My top three combos personally are Isla Charlotte, Isla Jane and Isla Harper. But I really want to use a literary middle name for my daughter, so those three are high on the list.
Isla June maybe? Pretty sure that doesn't fit the Scottish. I actually like Isla Jean. Isla Katherine would give you more play for nicknames. Or you could pick two (Isla Ruby Jean) then she'd have an option to drop one if she didn't totally love it
Isla Elspeth sounds more Scottish to me than Isla Elizabeth but both are pretty.
Isla Thistle - more symbolic of Scotland than actually traditional in Scotland as a girl's name. Thistle has a very Victorian flower-pixie sound to it as a name for me and Thistles are of course tied to Scottish imagery.
Not all Scottish but maybe something will appeal.
Isla Maeve was on our list at one point, so I vote for that one!
I love the name Isla, heres a few names that are popular over here in scotland that you could use as middle names!
Our daughter is Eila (same pn as Isla) Claire. We call her Eila belle as a nn but its not something we planned because Eila is super short and sweet to begin with. Belle has nothing to do with her name (clearly) so I think you can use whatever nn you want with your daughter!
I like Isla Beth. It's kind of like a play on Elisabeth.
First name I thought of was Isla Beatrice.
I love the name Isla Quinn or Isla Maeve.
Either way, Isla is a gorgeous name that your daughter will be lucky to have!