Making the icon set itself would be super easy - just gotta get the icons, convert to PNGs, and call it a day... apart from batmeter - batmeter gets special texture cropping treatment. The icon theme structure will obviously be hardcoded at the end of determining the icon sizes, and it'll only be the essential icons for the pre-installed core applications, for KDE Plasma, just as Microsoft intended.
For folders, mimetypes, and application icons in Vista+, allow for small icons to be put into the "yo icon too darn smol" border of the respective icon set for any sizes they are too small for.
For everything else however, especially actions and whatnot, just keep those exclusive to the sizes they do fit in
For -symbolic, either don't supply them period or just use the already conceived strat of just... shoving them into image-embed-SVGs.
Below this is just some ideas for icon substitutions:
Windows 7 and below - Airplane Mode: The 'no network connections available' icon
Windows Vista and below - Wireless Strength: The 'both computers lit' icon for 5/max strength, 'left computer lit' for 4 strength, 'right computer lit' for 3 strength, 'left computer lit' for 2 strength, 'right computer lit' for 1 strength, and 'no computers lit' for 0 strength.
Windows 8.1 and below - Mobile Network: Just use the Wireless Strength icons (that's what Microsoft officially did. )
Windows 8.1 and below - Bluetooth: Just... use the Bluetooth DLL's limited set of available icons, ig?
KDE Connect: Pretty sure there's a phone icon somewhere in each version of Windows starting at least with Windows XP
Desktop Effects - Windows Vista - 11: The Screensavers CPL icon?
Colors - Windows Vista - 11: Just take the L and use the Color Swatch preview and Color Swatch: Sky textures respectively (for Vista, alternatively use the Window Colour icon)
Wallpaper (Plasma 6) - Windows Vista - 11: That icon Paint uses, ig, in 7 and onwards...?
Application Style - Windows 7: Windows Basic icon...?
Icons - Windows XP - Windows 11: The folder icon
Font Management - Windows XP - Windows 11: Fonts folder icon
Splash Screen - Windows Vista - Windows 7: The Getting Started "Welcome to Windows" icon
Task Switcher - Windows Vista - Windows 11: That ALT+TAB icon in Quick Launch (it's in imageres.dll)
Desktop Session - Windows XP - Windows 11: The Shut Down Computer icon in shell32, since there's no other shutdown icon surprisingly.
Making the icon set itself would be super easy - just gotta get the icons, convert to PNGs, and call it a day... apart from batmeter - batmeter gets special texture cropping treatment. The icon theme structure will obviously be hardcoded at the end of determining the icon sizes, and it'll only be the essential icons for the pre-installed core applications, for KDE Plasma, just as Microsoft intended.