I'm kind of curious what sets it off from the others. What is it used for? Why will it be used? Who would use it?
All three criteria must be answered, preferably detailed or summarized. The bbcode has to be something the majority would use aside from other options, otherwise it's just a waste of bytes and takes up space in the script.
I mean, I personally don't see a use for left-slant. A quick search even confirms as far back as '08 that a custom font type is needed to be used, which would mean plugging the metatag in to call the font over, which would strain the server load, either by a margin or a wedge.
If you're talking along the lines of something like
this for the bordered text, this would be a table, but a table that increases in width and height in accordance to the paragraph itself. I personally see that as an eyesore on web pages, because you'll have things such as this:

That^ in itself is a web design no no. It's overly abstract, and very unorganized. That's why I don't like the idea.
Also, fulltable is literally the html you put. BBCode calls from a script that plugs html into a post.