Hey this is a nice project, I like to see this! Indeed this can't all be done with a plugin, because the API doesn't let you change shaders and other low-level rendering features. So this is a good project for people that want more features. You may even want to add the features that my GZDoom Editing plugin has (many UDMF stuff such as flat rotation/scaling/offset, individual high/mid/low texture offsets and auto-align, slopes, etc) Keep up the good work! And if you have any technical questions, feel free to send me an email.
EDIT: Oh I just read that you don't use SVN... that would really be an improvement in your work! I can give you a branch in the same repository that DB2 is in and you should start off this branch with a copy from one of the base revisions (latest 1483?) and apply your changes from there. This is important, because then it is much easier to apply any changes that I make to the DB2 base (and I'm sure there will be changes that you would like to have as well, such as bug fixes) and with SVN you can just select my changes and apply them in your branch (may need some conflict resolving in parts where you changed code yourself, but you know your own code so you will know how to work that out). Kaiser did the same for Doom Builder 64, which is also a fork of DB2 just like your project. You can find his branch
here and some more info
here. Let me know what you think and if you like this then I'll make a branch for you!