I think it's nice to have links to different branches of the family tree in a child's name, even though you can't keep them all indefinitely. It would be great to have your mother in law's surname in there as a first name, especially if you like it.
A few years ago I started researching my ancestry and I found it really striking how many names there are as soon as you go back just a few generations. Even if you double barrel your child's name, only two out of the four grandparents are represented.
But I did find it sad that I was unable to trace several branches of my family tree back very far because a woman's name was lost somewhere. If you don't know what her birth name was, you can't go back any further. I did my family tree just after I got married and I think it factored in to my decision not to change my own name.
My son has a version of my surname as his first name, my dad's name as his first middle name, and another name which is a family name from his dad's side but also a masculine version of my mum's name as his second middle name, e.g. George after his paternal great grandfather but my mum's name is Georgina. So I'm happy that his maternal line is well represented.
Currently pregnant with a daughter and we are struggling a bit because I want her name to represent her family as much as my son's does.
My middle name is one which has been used down the maternal line for five generations unbroken. When I first did my family tree I thought, that's a bit arbitrary. Of all the women in the family tree who gets to have her name passed on indefinitely, why that one? I was considering bucking the trend and if my son had been a daughter I don't think we'd have used it. But now I've come round again and am thinking that passing a name down the maternal line for six generations is a very powerful thing and I don't be the one to break it. So she will have that as her middle name, plus my other in law's first name, and we just need to decide on a first name.
In many ways I think the choice of first and middle names is more important than the surname and if you actually do your family tree, you might be surprised to find that given names have more staying power than surnames over the generations.