Here's one I designed and made featuring bows (bowtie?) and hearts, suitable for both gender. Yes, I used thirti3es reproductions, but any style fabric will work well.

Jan in VA

What a cute quilt Jan!!

I am thinking chevrons. very neutral, semi modern, add some grey chevrons for a nice neutral.

There is a Riley Blake line, I think, called Giraffes Crossing. On target for color scheme and gender neutral.

I like most of the ideas presented. I only suggest using Batiks for the quilt, especially since they don't want something babyish and gender-neutral. They read many times like solids or tonal but they look richer to me as well. When I make baby quilts for people and they want something not babyish I usually go that way. Just because they can grow with the kids. I am making two right now for my baby girl who is going to be born in October.