In another world
I'll be your cheval glass.
when you smile
I'll be smilling back,
and when you titter
I'll share your joy.

First of all, i love the title you chose for this piece. It is very good since what you talk about is 'If' that were to happen. But it can in another world. You will contribute to her happiness by smiling back, by being there for her.

in another world
I'll be your best sweven.
that which you'll wish never to fade away
that which is but impeccable.

You used good words here and you knew what to do with them and how to put them. You will be her everything, very sweet.

in another world
I'll be your blanket.
I'll cover you with my embrace,
in me you'll find warmth.

you will be her comfort. You will be there to keep her warm and make her feel safe.

in another world
I'll be your favorite song.
the one you'll always replay,
over and over again.

This was my fav stanza of this poem. How you used a song to portray yourself was genius and so original.

in another world
I'll be that star;
upon which your wishes
will come to pass.

You will again be her everything, you will provide her everything, very good assurance in this stanza, beautifully written

in another world
I'll be me.
always loving you,
always and forever.

Nice ending, you will always love her, too bad this isnt the other world, or is it? Great write, you are talented.