I'm in agreement with OP about most of the changes to add back things from PoR like BEXP and in particular the base conversations. I feel like the last few games have been rather weak on lore/making the in game countries feel like distinct places and not just named places that have a vaguely differing Aesthetic, or whatever. In PoR the player gets to see firsthand the differences in ideology citizens of these places have (ie prejudice or lack thereof against Laguz, how commoners can move up in the world, even mentions of past wars and economic strife) through conversations with unnamed NPCs, which, realistically, would be the people most affected by the events of a huge war.
Fates was deeply troubled in terms of a consistent narrative, and I'm hoping that the switch title will bring some better worldbuilding to the table than both it and Awakening were able to manage (Awakening's primary narrative weakness in my eyes being the Mustache-Twirling "Let's end the world because evil" logic of the grimleal). Echoes did better than the both of them, and maybe could have achieved something like the base conversations if not for the fact that all the NPCs really seemed interested in doing was giving Alm and Celica intercontinental fetch quests (they were enjoyable for me, but not exactly enlightening in terms of story).
Gameplay wise, I can really only echo previous opinions:
Bows with 1-3 range and drawbacks for shooting at 1/3 range as needed
Make forging less broken or give it a better utility like limited weapon repair
bring back diverse map objectives and gimmicks, if only for BEXP. one that comes to mind is the PoR chapter "Solo", AKA priest-shoving simulator. Time consuming as it may have been, trying to force your way through a map without killing certain/any enemies (this also applies to the desert map of PoR with Tormod and Muarim's escaped slave crew) is REALLY interesting because it forces you to change your strategies from "throw a Lord at it until it dies". And lowkey offers built in time to heal grind, but shhh lol
Possibly less popular, but I really liked the concept of mages learning their spells instead of reading them out of breakable books. I wouldn't mind it if they kept the magic system the way it was in Echoes, but if they do change it back I agree that light magic should come back, as well as magic effectiveness similar to the -slayer weapons, ie thunder > dragons, fire > beasts, light >undead (assuming there are undead, as has been the trend lately) and etc. Basic 'starter' magic could be type-less so units don't start off with an advantage, assuming they keep the Echoes system of learning spells by level up.
I also agree that the extra modes like a randomizer mode would be fun and contribute a lot to the replay value!