I actually thought about reviving some super old topics for this, but my subject is only tangentially related to those topics.
So something multiple people have worked on was a double-walking-speed patch for Phantasy Star III: Generations of Doom. In particular, Tony H. had a series of Game Genie codes for it, and Tryphon actually made a patch that added both double speed and diagonal movement.
The issue was that the game's cutscenes that automatically take control of your character weren't designed for double-speed, and tended to screw up. Fortunately, in the last couple years, a full disassembly of PSIII was released, and from that, I managed to take what was learned about double-speed and combine it with the intro cutscene to create something that at least doesn't require you to turn codes off and back on to work properly.
It does still look a little weird. The specific pathing taken is due to the fact that, while I can work okay within the existing contraints of a ROM, adding new things/instructions is currently beyond me, even with the disassembly. As such, I had to work within the max number of inputs the original cutscene does (To oversimplify, the game-takes-over-for-you cutscenes are a series of "Hold this direction for this many frames" instructions.). Further, if you get too far ahead of Maia, she simply stops moving...but she's a solid object, so if you get behind her, that screws things up too. Still, I think I did all right. A demonstration is here
; you can see the cutscene playing out after a brief demonstration of double-speed. An earlier demonstration where I got it into a barely-working state, that continues through to when you exit the shop after escaping the dungeon, is here
If you want an in-lore reason for the movements, I like to think of it as Rhys being so deliriously in love and impatient to get married that he temporarily forgets where he's supposed to go, then impatiently keeps stopping and waiting for his super-slow wife-to-be to catch up, before finally having a chat with his guard bro just before the ceremony starts.
I know there are a few other cutscenes like this throughout the game I'll have to play through and see how they react, but this is encouraging so far.