Fire Emblem: Souen no Kiseki (aka Path of the Blue Flames, aka the one with Ike before he started pumping iron)
Despite having a giant backlog, I somehow got ensnared by this. Decided to give Maniac Mode a go. Patching a JP ROM wasn't that bad (definitely don't get it pre-patched, that's asking for trouble), too.
Now, if you search up Maniac Mode, you'll find it has a.. let's say.. mixed reputation from the first Google hits. It's more about overwhelming you with extra numbers, which can actually work for say FE7 Eliwood Hard Mode if you care about your XP rank and are training lowbies constantly, but won't really challenge immortal juggernauts that much. Also, Zapp Brannigan style "overwhelm your kill limit with waves of attacks" is actually not totally crazy in a game with expensive forging - those extra enemies will dull Titania's axes in all the blood. Really. As usual, the earlygame is the worst on this - by the time Muston / Jorge / Daniel / Aimee showed up with their shop, I'd broken Titania's Iron Axe and Steel Axe, Ike's Iron Sword & Steel Sword, Soren's Wind tome, Oscar's Iron Lance, Rhys's Heal staff (only 30 charges under JP rules!), and Boyd's Iron Axe. At least Boyd still had a decent number of uses left on his Steel Axe, Poleax, & Hammer, but… yeah, the extra enemies are notable.
The other notable change is to the XP rules. You get slightly less XP for kills. But! The enemies are a bit higher level which gives a bit more XP to offset this, chipping XP isn't affected, and there's the aforementioned waves of enemies, so you get plenty of battle XP. Bonus XP really is noticeably cut, though - you get enough to rig level-ups at the base if you want, but not enough for easy major catch-up. This has the unfortunate effect of making objectives and efficient play matter less, and going on massive killing sprees matter more - trying to rush through the river escape map (C6, "A Brief Diversion"), aside from being suicide, just doesn't pay off. You're going to have to take it slow and steady and murder everyone and eat a bonus XP penalty for not speeding through, but who cares, because the bonus XP wasn't that much. The even starker example is Feral Frontier, where slaughtering all of Muarim & Tormod's rebels for battle XP is 100% the play now rather than attempting to spare any. (This was always true, I guess, but it's MORE true on Maniac.)
This isn't all bad (and arguably bonus XP needed a nerf anyway). The extra toughness on some enemies does reward things like Soren's Adept as more relevant. Plus, at least based on Elf's old writeup, I think enemy AI changes a bit, with fewer stationary enemies - it's been far too long since I played FE9 North America, but the feral Tigers in C14 definitely move, the mages on Norris's ship in C13 (Astrid map) definitely move and can be quite annoying, and so on, which Elf's writeup seems to indicate that they don't. That said, Maniac Mode's boosts certainly aren't enough to deal with the most famous FE9 exploit, that of Earth / Earth supports, so you can still make a nigh-unkillable Ike & Oscar cooking duo if you want.
Other thoughts.. Shinon is worse in JP rules which, unironically, makes him better. I'm serious. JP Sniper doesn't have the +15% crit bonus so he's actually very good for chipping to set up kills for others, and during the very dangerous early turns of C4 ("Roadside Battle", Soren's map) he can still facetank and build walls to protect Soren & Rhys, which really helps with the extra enemies swarming you. Titania is just as amazing as usual, act shocked. The main pleasant surprise has been Mordecai - having a pre-promo tank who can hold chokepoints with no training necessary is shockingly relevant on a decent number of high-deploy count maps (C11, C13, C15), especially if you're benching some early joinees and planning to use later recruits. Do you want to keep using him in the second half of the game? Hell no, but he's an authentically good unit on some tough missions.
I'm up to C16 currently, "The Atonement." Enjoyable enough, although I am reminded that FE9 can take awhile to play itself in the backline by modern impatient standards, even when combat animations are turned off.
And oh yes, I still really like the plot & writing so far. The only nitpick I'll offer, and it's not a big one, is that Ike does a "voice of the player" moment around C8-C9 where he basically acts like he doesn't know anything at all about Gallia or the beast tribes, either in favor of tolerance or in terms of imbibed prejudices. Now, yes, you-the-human-player doesn't know anything about this fantasy world and needs to have it explained, and it's also an explicit plot point that Ike is considered lucky in Benignon to have the privilege of growing up in a tolerant community that didn't ingrain certain strong class beliefs in him. So being a little naive and shocked at prejudice is fine. Not knowing *anything*, though? That's a little too far. Wish the writers could have cooked up a conversation where the parties cover all the relevant points for the player to learn but in a way that indicates Ike is sheltered but not ignorant.