There are romhacks, there are *good* romhacks, and then there are romhacks that take a 20-year old game to a whole new level. One of the first things I heard about Subversion was that a new SM hacking tool was created specifically for this hack. I couldn't not play it after that, and it absolutely held up to my expectations.
World design
One of the limitations I feel the hardest in most SM romhacks is the number of distinct areas. Sure you have sub-areas like Kraid's Lair, the three flavours of Brinstar, Lower Norfair, but some larger hacks often just feel like vast expanses of reddish, greenish, and blueish rooms without enough variance. Especially in areas that you have to spend a lot of time in, seeing the same tileset room after room and listening to the same music on repeat starts to feel almost claustrophobic.Subversion absolutely demolishes this problem by having over 20 separate sub-areas, all with unique tilesets, unique music, and even unique map tile colours. It even solves one of my usual nitpicks with hacks that use tilesets from the GBA Metroid games: all of the GBA tilesets are edited to remove the trademark red tile outlines which usually clash with the rest of Super Metroid's graphics. There's even a Castlevania (Symphony of the Night, I think?) tileset to be found if you explore enough. Some of the music might be custom (either that or I just don't recognise it from elsewhere), but what is taken from other games actually feels like it fits very well, including some of the highest quality recreations of N64 music that I've ever heard on the SPC700 processor.
The well-themed areas also have an element of well, world design, that really feels like it goes the extra mile in a way that most hacks usually don't. The hack takes place on a volcanic landmass riddled with underground caves and Chozo temples. There are numerous distinct biomes and facilities to explore; dark deep sea caverns, frozen mountain peaks, lush jungles, an orbital space station, a geothermal energy plant, and even the very heart of the volcano itself.
Item placement-wise, Subversion is neither disappointingly easy nor infuriatingly difficult to achieve 100% completion in. There are tactfully hidden items, but nothing you wouldn't be able to find using the X-Ray Scope and the hack's custom Orbital Scanner Log; a late-game mechanic that reveals the locations of all uncollected items on the map.
The room names are also a nice extra touch. They give the world more character, and are also a godsend for locating item hints in the hack's Archipelago Randomiser implementation (Have I mentioned Subversion has both a standalone randomiser AND an APWorld?).
Progression and difficulty
Some hacks do custom items better than others, and Subversion does them very well. Most items have subtle changes and are given additional utility in progression, such as beams having custom damage scaling and being required to destroy certain blocks, and the Gravity Suit (now changed to the Aqua suit) only giving increased water mobility and zero heat environmental protection. But there are also some very well-implemented new items in addition to the usual fare.New items (Spoilers)
The Gravity Boots are required to overcome TN578's increased gravity. It's perhaps the least exciting of the bunch, but the fact that you can unequip them at any time to fall at extreme speed is also a benefit.The Dark Visor allows Samus to traverse very dark rooms and see certain invisible enemies and switches. While you can still navigate dark rooms blind if you know what you're doing, the visor itself is required to interact with the switches, as well as to battle Phantoon later in the game.
The Metroid Suit provides immunity to extreme heat and Verdite lasers, but leaves Samus vulnerable to the cold, similar to Fusion Suit. It also has a secret weapon up its sleeve; standing in an energy source with the suit equipped allows you to fire hyper beams that can destroy certain blocks and damage certain enemies.
The elusive Hypercharge replaces charged beam shots with the hyper beam, allowing you to use the hyper beam anywhere, unlike the Metroid Suit. Its usefulness is well-balanced with the difficulty in obtaining it.
Damage Amps increase your beam damage by a flat amount.
Accel Charges reduce the time it takes to fully charge a shot.
Space Jump Boosts give Samus an extra midair jump.
In addition to these, Subversion also uses an ammo pool system, and most Missile, Super Missile, and Power bomb expansions are replaced with small and large ammo tanks, which give an additional 5 and 10 ammo, respectively.
Progression can sometimes be a rough point in romhacks. Endlessly backtracking and scouring areas for new items is painful, and when the intended progression is murky (looking at Hyper Metroid...) the gameplay just becomes a slog of never knowing whether you're on the intended path or sequence breaking, wondering at which point you've have to backtrack all the way to an elevator and try a different area.
Subversion does progression very well. It doesn't feel confined or forced, but the player is given just enough direction to have an idea of where to explore. You're even afforded some choice in which order you obtain certain items. You can obtain the Super Missiles and Wave Beam in either order, the Power Bombs and Ice Beam in any order, and more. You can even obtain the Super Missiles before Kraid if you think outside the box. And if you get stuck, the hack actually has a built-in hint system that keeps track of obstacles you haven't been able to pass that you can revisit once you've obtained new abilities.
The hack is very balanced difficulty-wise. You can play in either easy or normal mode on your first playthrough, and hard and challenge mode (more on that later) are unlocked on subsequent playthroughs. It took me just over nine hours to finish my first blind playthrough in normal mode, but only because the hack does actually have nine hours worth of content for an experienced SM player in a blind playthrough. It also isn't too difficult for a novice; you could maybe even complete Subversion without even having played Super Metroid; but at the same time it was never a pushover.
There are some items that are certainly more difficult to obtain, but what is a Metroid game without puzzles?
If you've played Super Metroid (Or even if you haven't this just sounds interesting), you should absolutely give Subversion a shot. It will even run on an actual SNES!