As an example, try choosing the RGB monitor option in https://dosgamer.com/games/play/microsoft-flight-simulator-2470.html and run the demo, or try playing https://dosgamer.com/games/play/round-42-3264.html or https://dosgamer.com/games/play/icon-the-quest-for-the-ring-1924.html

Of course there are many other early games written for specific machines such as pcjr, tandy, ega and vga that don't work right with the default svga_s3 setting, if at all. Plus, many games support multiple machine types, but look decidedly better with, say tandy or pcjr than with cga. MS Flight Simulator 2.0 above also supports pcjr and looks great in 16 colours!

To solve the machine type issue, a drop-down of the various machine types (cga, hercules, pcjr, tandy, ega, vgaonly, svga) could be added that would run the emulation using the selected machine type. When this machine type value is changed, it would be best to stop the emulator if it's running, forcing the user to restart it with the new setting.

A bonus feature delivered with the ability to select the cga machine type is CGA Composite Support. A lesser known feature of dosbox, pressing F12 in many cga games will switch to composite colour mode, which blends pixels horizontally to deliver more colours at the expense of some blurriness.

After implementing the machine type selection feature, cga composite mode should work, and we could even add a toggle button to switch it on and off when machine=cga is selected.