Growing up, meatloaf was 100% beef and meatballs were an even mix of beef/pork/veal. I know a lot of people who still like the triple blend, but most have settled on just beef and pork.
Why? Because you used to be able to go into any supermarket in New York/New Jersey and get that blend already made up in the meat section. But as veal prices skyrocketed (and some people look down on veal consumption...) veal was taken out of the mix at most places. Of course you can find any blend you want at the good butcher shops...
I go straight beef for meatloaf. If I want some pork, I'll just wrap it in bacon.
https://www.pinterest.com/pin/158963061825303822/
There's no right or wrong for meatloaf. Some put cheese in - some put a hard-boiled egg.
I just season it well, I use milk-soaked bread cubes (preferably buttermilk) and a decent amount of ketchup and milk in the mix - keeps it nice and moist. Some use ketchup on top as a glaze. I do that sometimes, but not too thick. And a nice glaze with BBQ sauce is also nice...