Elena - nice but a lot of El names around at the moment and lots of Ellies. As someone suggested, Eleni is nice, then there's the shortening Leni which I really like.
Lea - I really like all versions of this like Lia and Leah etc. It's a beautiful and elegant name. But there are a lot of short L names about at the moment too like Lila and Lena and Luna and Lara and so forth so would it feel a bit lost in amongst them?
Sasha - I don't love this but maybe as a shortening? I like that I haven't seen it on here much so sounds quite fresh.
Natalie - I really like Natalie and it's a bit more distinctive and nice to have some consonants. I also like Natalia but that's quite long.
Mischa - I like this. I think Sounds a bit like Sasha but I like it better because of the sound being a bit more definite with the M.
Isabelle - nice name but very popular and has been for a while so doesn't feel so special or distinct at the moment.
Mari - I like. Quite different in style to the others for some reason?
Poppy - I'm not too keen on Poppy. It's ok.
Zoe - not sure quite I'm not more keen.
Alisha - it's ok. Not my favourite.
Cleo - I like Cleo. I also like Cleia.
Helena - I do like Helena. At least it sounds a bit more there and not quite so vowelly. But it sound sound a bit upper crust and a bit formal to me, but maybe that's the Helena Bonham-Carter association in my head. She's a great actress though so that's not a bad association at all. But I do feel it's not easy to shorten and therefore sounds quite formal, which I wouldn't like to have for myself.
My favs are probably Natalie, Mischa and Cleo as they sound more distinctive at the moment. With Eleni, Lea and Helena runners' up.