Ooooh, what's my old laundry list?
I definitely wanted to go back to the cinema a few weeks after ST:TMP to see the next new story. As I began to learn about "Phase II", and bought and read some scripts that featured Decker, Ilia and Xon, I missed those characters so much. Similarly, the loss of Kirstie Alley's Saavik - who turned out so much better than the pointy-eared Mary Sue character the newszines were panicking about, stymied plans for younger characters to start replacing the older characters as the focus of storylines.
Chapel and Rand are in the novelization of ST II. I know Majel Barrett withheld her services for ST II, but poor Grace Lee Whitney wasn't even contacted about reprising Rand. And they did use the transporter in ST II. (Grace actually attending a screening of ST II in Brisbane, Australia, because she still hadn't seen it in the US, not having been invited to ST II's gala premiere.)
I missed the TMP uniforms a lot. I was shocked during the "It's a Wrap!" Paramount auctions how many unused costumes had been made for the Epsilon 9 scenes! Roddenberry could have done a space-station based TV series in the early 80s with all the space station uniforms - and the amazing ambassadorial alien robes - that were made for TMP and hardly glimpsed onscreen.
I recall a "Best of Trek" article that suggested a great future villain might have been a rogue Andorian. I did get fixated on that idea! I imagined John Phillip Law (the blind angel, Pygar, in "Barbarella"), in blue makeup and moving antennae, as a charismatic Andorian, either good or evil, I wouldn't have cared. (I guess I was anticipating Shran in "Enterprise" decades too early! I still can't believe how lucky we Andorian fans were with that series! Originally, the plot of "The Andorian Incident" had been written with Gorn in mind!)
We definitely could have revisited the TMP alien races in future movies and TNG. More than just a few re-appropriations of the TMP costumes that we only glimpsed in ST IV and VI. And yeah, Arex and M'Ress as regulars. And more Kyle after ST II! (What a delight to see his return!)