I have located the video output on the non-campible games with VGA, and I think maybe they could be easily fixed, but my little experience allows me to move forward ...
I have tried with the game Deadly Sky Pal Multi5, and I have found this
0.000 (1st_read renamed)
offset 0x170ABC
It seems that this function is the one that takes care of the video out, and best of all is that it is found in all games, although in some they change their values
These are the changes that I have done and this is the result
0x170BE4 Change to B0 FB 9A 8C
0X170BEC Change to B4 FB 9A 8C
Result



The emulator game appears as VGA 480P 60Hz, but only half screen is displayed on the left.
In console the screen appears in black and only sound is heard

This could be the beginning so that all games can be fixed, although clear is that you still have to change some more values

I thought maybe it could be possible a method easier in this way, but that something else was missing by polishing.
But I guess you have surely done tests with this

In the game Plasma Sword Pal, I got the full screen but the Hz goes down to 30Hz

KOF 99 DREAM MATCH
I only get 480p 60Hz on 1/4 screen and 480p 30Hz on 1/2 screen



Kof 99 Evolution I can not try it froze in the sega logo

I have researched the subject a little more and I have located the video output in RAM
VGA Standard (these values you will find them in any game compatible with VGA)



Now I will show you the original values of KOF99 Dream Match



As you can see in the photos I have left you marked the possible values that I think you have to modify
Modifying these values ...
1st_read
0x18B954 to 00002044
0x18B958 to 0000F043
0x1B1DCC to 405AA98C
0X1B1E20 to 8002E001
With this I have hacked a bit for the VGA exit but it is not complete yet.
These are the values that are now shown with this small edition



Result in Game.
480P 30HZ 1/2 SCREEN




I think it's worth investigating

Ahh ok, I understand.
I have tried what you mention and are reset to the BIOS menu.

But this may be because the VIDEO MODE or HZ Mode code is not well implemented

I know what causes the problem.
Is the resolution 640x480
I have tried in 256p mode to raise the resolution at 640x480 and exactly happens, it is reset to the BIOS menu

I mean the game has a problem with the resolution and not with the VGA mode
You can try and you will see that it is the resolution
0x1EFEC2 to 8002
0x1EFF42 to E001

The game normally starts but reset to the BIOS menu by selecting character, just as it happens in VGA mode

There must be somewhat some kind of resolution limitation, and the game when verifying that it is not your resolution is reset

The games Plasma Sword and Deadly Sky do not have that problem, so maybe it would be simpler to be able to launch a VGA FIX.

This maybe you could reduce the list of games not compatible with VGA, and noting those who have another type of problem as KOF99 DM.
With Kof 99 Evolution I have not been able to try when freezing in the logo of SEGA