Honestly, part of the thing I like about iOS is how similar it is in form and function across devices. It makes for a more seamless experience from mobile to "laptop" when going from my iPhone to iPad Pro. Split screen and PiP for video are pretty decent differentiators between iPhone and iPad experiences; what more do you really want or need? Apps in windows? At that point you start getting into MacBook territory, which admittedly Apple may be considering, but probably not yet.
I love the iOS experience on the big screen, and though I'm not ready to give up my iMac yet, my iPP has been a perfect laptop replacement for me. My poor rMBP sits mostly unused nowadays.