Trying to refresh my skills using DASM , I started to make little tweaks to 2002's Koffi: Yellow Kopter.   So far, did these things:

• removed y2k-compliant bootup code (the Atari fuji symbol w/multicolors). Freed up some ROM and the game boots immediately to Koffi's title screen now. 
• modified the hues of the raindrops to a more blue hue. Using hues $B4/$B8, the raindrops looked green on emulators although looked bluish on my orig 5200 and on ancient emulator VSS.   NTSC color values weren't what I orig thought they were. Well the raindrops are blue now.  
• Fixed a bug with Koffi's water tank level.  It was challenging to find the reason why the yellow water meter would display as though it were full, but internally the water value would wrap around to $00 value (thus no water untl you catch more!). It could happen (usually on Easy mode)  when you had a full meter, after Pyro the cloud was defeated and the final countdown was going on to put out the fire.  Well, I found the cause. Fixed.
• You can start the game with a Joystick button press now. 
• Fixed another bug on Pyro's Mom (the 4th stage which is a boss fight). If the fire Chaser gets hit by Mom's lighting, he shoots up and hits her, stunning her and giving you some time to safely go hit her right in the clouds. (ow!)  The bug , which I knew about but decided back then to call a feature, is the Chaser can get stuck in this sequence in an infinite loop.  Solved, fixed. 
• Tweaked the graphics for Mom's eyes after she is defeated to be more centered as I originally intended.
• Tweaked the Tree Bonus logic . It is easier to get the 500 point max bonus now, and I added some more granularity to the points levels.  
• Tweaked the settings as the levels increase. You can defeat Pyro a bit quicker on each level now, speeding up the pace of the game a bit. 

Reviewing the code, I believe I can free up some ROM space and make some more gameplay and graphics tweaks for Koffi 2020.   What I'd like to try is to have a 2-player simultaneous mode.  I always wanted that (like in the Williams game Joust), but it was beyond my abilities back in 2002.  We'll see.