Well, Driver modding is widespread by today...

Everyone can mix&match the best Files needed to his/her private purposes...

But remember this will not be a really 'new' Driver, just one among dozens of others...

There's really no legal obligations involved. Common "Netiquette" is, however, to at least give Credit to the person which Driver you (re-)modified.

Making a really new Driver from scratch is close to impossible, because of one big reason in the beginning :

You need the SourceCode.

This might mostly be in C or C++ (or comparable), with Assembler parts for the most time critical routines.

And eventually, you'll have to be pretty good at Programming, together with the need to understand how a Win32 Driver works.
It will be both the Win95/98 Driver model, and the later WDM Driver Model (Win98SE, ME) and with some changes the Win2k/WinXP Driver Model...

I have a Link in the Technical Resource Page to a pretty good guy, who did some Assembler Coding for a Voodoo3.
I believe he used MS-DOS though, and didn't make anything close to Driver programming. But might be a useful address for a start.

Free sourcecode you will only get in the Linux world, where some full sources are existing. Still, that's from another Operating System, and will be quite different from what is needed to create a Driver for the Win32 envroinment

Greetings
FalconFly