I think the only things salvageable in their entirety from G1 are the villains. Grogar I believe needs no introduction, and for as washed out as his color scheme was Tirek was also very awesome. Scorpan is the only villain in MLPFIM who actually was intended to be interesting, having an actual Face Heel Turn as part of his narrative. Katrina COULD have been more interesting in the right hands, and Lavan was also kind of cool. Nastina and her daughters can be tossed in the garbage, but the Smooze was cool in a “mindless nuclear chaos” kind of way.
I also believe they need to bring back the Sea Ponies (as an analogue to mermaids) and Flutterponies (as an analogue to High Elves/Eldar), and maybe bring the Bushwoolies along so long as they are treated like shit (as an analogue to Fluffy Ponies. XD)