If you're stuck on what to use as a middle name, then maybe it's best not to have one. My little boy's is Samuel, after my dad Usama (Sam) and my OH's dad Melvyn (Mel) - Samuel was the closest thing to putting them together.
If we ever have a girl, her middle name will be Kathryn after my Mum, OH doesn't speak to his, so no arguments there...
I think middle names are non-essential, but a great way to pick a name you like as the first name and still honour family members...
Picking up on the comments that a couple of other people have made, I have three brothers, an older half brother from my Mum, a full brother, and a younger one from my dad, and they all have middle names, and I don't! My name is quite long, but I'm so jealous, I'd love a middle name!
So I agree with it not being fair if a later sibling gets a middle name haha!