Okay so i'm just about to reach Heret Kingdom in Lufia 1 and I have a few notes in case you ever wanted to patch.
In game, Amon is repeatedly referred to as the "Sinistral of Terror", despite him actually being the "Sinistral of Chaos" Also, It might be worth dropping the encounter rate and increasing the time that Sweet Waters works. Having increased run speed helps a lot but the encounters often happening every 5 or so steps is so painful and ultimately bog the game down with an overwhelming amount of encounters, especially on the ocean. I know this is just a thing that used to happen in this era of RPGs but it would be a real quality of life improvement if encounter rates were lowered. Lastly, perhaps having Retry Mode available from the beginning would be nice touch but ultimately unneccessary. I'm playing on NG and there have been a few times where it has been very very grindy.
I'm a few episodes off Heret Kingdom in my Let's Play (Currently ongoing. Doing them in chronological order) but you can see it here - https://www.youtube.com/watch?v=O8jq-T82mjE&list=PLVFlHPy0Pa9tsPbSGCQbVJBA2uh5fhEBq
If I were going to add anything to Lufia 1 it would be adding darker outlines to the sprites to make them more closely resemble the sprites from Lufia II and maybe add some dialog somewhere with the main character referring to his father 'Jeros' As much as I could dig, the closest thing to a canon name for the hero was Nova though that is questionable - https://lufia.fandom.com/wiki/Maxim%27s_descendant
"The screenshots in the English game manual refer to him as "Nova"."
Perhaps adopting that as the closest thing to a canon name he could be referred to as that name in the intro sequence to Lufia 3.
"The Hero is mentioned in the introduction as as Maxim's descendant and the leading figure of the 2nd Doom Island War. In the description, he had to sacrifice Lufia, the girl he loved, in order to save the world."
So instead of Maxim's Descendant, It could be "Maxim's descendant, son of Jeros, Nova" just to tie in the family line. I dunno..
Lufia II's Let's Play has long been complete but I did have a few notes. Overall it's perfect. Having Retry available from the beginning would be nice as you'd still have Gift Mode to unlock but again, not necessary. A few NPCs had some oddly worded dialog but it's all solid as a whole. Some end game stats and the credits in general were kinda broken though. Aside from that, it's the best way to play Lufia II hands down.
My full Let's Play is available here. It should be 100% completion - https://www.youtube.com/watch?v=_WnczsKIuVI&list=PLVFlHPy0Pa9thA-EoXtEMn-dO16S-NR-T
It'd be cool to add onto the game a few things like using the Japanese versions variant puzzles somewhere, like the Sword Shrine and Ancient Tower's. The Sword Shrine's could serve as an extra room and the Ancient Tower's could simply be added back in. Based on what The Cutting Room Floor said about the Ancient Tower's cut puzzle, you could allude to the solution with a plaque on the wall fairly easily. Adding in the unused prototype tracks would be pretty cool too. Gades has a different sprite in the prototype as well so it'd be cool if the first time you fight him if that were the used sprite, though the feet would need some work (i'd be happy to fix them up)
From what I gather, the Doom Island map is traversable beyond the "We can't turn back now" dialog within the final area of the game. A cool thing to add in would be to add collision to said map, make it traversable and to REALLY tie things up, create a small map for Arek's shrine at the bottom, populate it with final floor Ancient Cave enemies that are unavailable elsewhere in the game and make the aim to open up the warpzone at the bottom of the map so you can return to the world below making the final area somewhere you can leave. You could add a small warp shrine in somewhere that requires a key for entry that could be obtained within Arek's Shrine and you could add in items like the Old Sword/Armor/etc to make it worthwhile.
Lastly, if the above were added, I feel like you could get away with making Tia, Lexis and Dekar selectable characters in the world below but script it so that you're forced to take Selan, Guy and Artea for the final battle. It would give players 3 other characters to equip and max out that they could take into the Ancient Cave whenever they felt like it. Gift Mode would still have a point as it's just the Ancient Cave really but this would overall open up the gameplay, customization and give more characters to equip/max out. All of this would take a lot of work no doubt but it could be accomplished without taking too many creative liberties.
Lastly, I obtained all the Iris Treasures and you pretty much get a high five as your reward which is kinda lame. If an equipment/item set pertaining to each Iris Treasure were created that you can take into the Ancient Cave and you only obtain them once you get them all Iris Treasues. That would make it worthwhile. Maybe make the Master Slime allow for switching Capsule Monsters out of Master form without fruits. Maybe make the jar a reuseable Hi-Magic or something.. I dunno, it's just a long winded bunch of outlandish suggestions that could be seen as a decent add-on haha
To be honest, there were far less Blue Chest treasures then I remember. It'd be cool to have more. Maybe an Idura Rapier with Plasma Blaster or Idura Armor with Idura Thunder and stuff like that.
All bugs and ridiculous suggestions aside, I will be doing a full run of Lufia III after 1 to complete the Trilogy (Ruins of Lore is dead to me XD)