I came to a thought that we should use new thing flags to allow make maps for several game modes with some difference in things lying around for each mode, since tactics may differ.
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Basically we may use flags starting from 0x800 (2048), but this would require substituting some parts of the common progs (this is possible now), precisely the one which handles reading flags when spawning a thing during game, because there are some extra flags mentioned; probably they are used for compatibility with other ports, and not needed for KA at all.
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Deathmatch flag is the most important, I propose rename it to "Netmatch" just for convenience (but it will have all the same function in game). Then, other flags should be (at current point):
0x1000 = Capture the flag
0x2000 = Domination
0x4000 = Castle sweep
0x8000 = Football
0x10000 = Xmas Havoc
Since DoomBuilder allows to customize thing flags there will be no problem with it.
For other editors, like WadAuthor, I suggest using Single Player flag as "All modes" or "Always", that is if Single player is checked it will automatically count as all modes checked.