I actually have created this account just for this forum, lol, but I'm going to mention a majority of the things I've noticed through out my single player and multiplayer playthrough that will mostly be opinionated so take it as you will. (Both playthroughs are still pre-hardmode currently)
Single Player - There's actually not a lot to talk about here but as mentioned balance changes.
X-ray Scope - In my opinion, X ray seems pretty overpowered, I'd suggest Instead of covering a majority of the screen with light, instead only light up a small ray, very much like Super Metroids x ray scope worked and have it follow the mouse as it already does, simple stuff.
Missiles - They are extremely powerful especially with a missile station nearby; this could easily make getting every expansion obsolete. My suggestion to help this issue is to give missile stations a 6 minute cool down, so you can't use one again until the time is up.
Also the missile pick ups are a nice touch, however when your inventory is full you can't get anymore pick ups, it's just a small nitpick to me but adjusting how they work would be great.
Morph Ball Bombs - I love how you can bomb jump just like in the Metroid games but the bombs feel pretty weak as weapons themselves, I just opt to get out of morph form to deal with an enemy, but this could be your intention I'm not sure. Just simply buff morph ball bombs and give them extra damage where it's needed.
Recipe - For the Spider Ball you need either 8 Palladium / Cobalt bars, Emerald and 5 Spider fangs. But why when you can already find spider ball from chozo statues already? I recommend changing the recipe to pre-hardmode materials.
Chozo Ghost Npc - I like the idea and design of this npc but what it sells simply lacks, all he sells is a single boss spawner which will quickly make this npc obsolete unless you need extra ores from the boss. I'm sure there will be more items to be sold from this npc in the future but for now it's quite stale.
Torizo - It's way too easy to cheese this boss, and this on it's own could make fighting king slime / eoc almost a joke once you have the power beam from crafting it. There's not a whole lot you can do honestly but at least adjust his jump so depending on your height he will be able to reach you, as at the moment you can just build platforms above him and he'll never touch you.
Serris - This boss feels really unfinished. Maybe that's how it is in Metroid Fusion but it just feels too easy. I don't know how you would edit this boss fight in anyway but giving it more challenge would be a great idea.
Multiplayer - So it was stated that enemies and bosses have wonky behavior in multiplayer but so do items that are placed like Missile expansions and the chozo statues itself. If you try to break it normally / right click it you won't even receive the item which was a massive pain when we went to the ruins and got nothing out of it aside from the carpet and sandstorm. If there's anything in multiplayer that needs to be fixed it's this.
Bosses not spawning - When we tried to summon the only 2 bosses from this mod in pre-hardmode we had issues, Torizo would sometimes not even spawn even though we used a summon. And the first time we tried to fight Serris it just went off screen and despawned, wasting a summon.
Grapple Beam ?? - I said this on my last post about this glitch where you grapple a platform, hold up and jump. Please fix this when you can, I want to be able to use my grapple without worrying about being zapped into space.
My own personal ideas - These are purely my own suggestions and stuff I'd absolutely love to see in this mod if it isn't planned already.
Adventure Maps - It's no surprise that people are going to want to make adventure maps using this mod and I'm one of those people. If you were able to craft bomb blocks, speed booster blocks, screw attack blocks, maybe even beam doors and most things that are in the metroid titles.
More Naturally Generated World Structures - The Chozo Ruins is a great start, but I would love to see more areas like this, it doesn't even have to be special or anything but considering the huge boss gap between Torizo and Serris maybe once the player obtains missile they can access a certain building by breaking open a red hatch there, just like progression in the metroid games!
That's all I have for now but I will update much later when I have more info / ideas. Once again thank you for making this awesome mod!
Also add Crocomire as a boss thanks XD