Mybb community, very immature.

[ZiNgA BuRgA]

That's an interesting point you make there.  But essentially, I don't think modified code will be blindly copied across, rather, it will most likely be manually merged, and since ideas don't fall under copyright/copyleft, you can easily get around any license restrictions that way.
So in effect, even if you fork the LGPL project and make it GPL, changes made in the fork can go back into the original project, as long as the developers can show that the code wasn't copied (i.e. modify it enough to make it perceptually different).  But this is a bit more difficult I guess, than simply copying changes across, so does provide some protection.

There's also the question of what your fork is exactly trying to achieve as well.
Yes, there are many issues in MyBB which can be fixed, but unfortunately, MyBB seems to primarily target the majority webmaster community, which is mostly made up of people clueless about these issues.  One of the reasons why MyBB is successful is due to the overwhelming number of features it has for a free forum software.  Even if there are issues with these features, the fact that it "works most of the time" will probably be sufficient for the wider community.
The issue here is that trying to promote a bug-fixed fork of MyBB will be difficult, if you plan to gain a fair bit of a following.  You're probably better off trying to develop new features (and spend less time polishing them) to gain attention.

That's from a development point of view - there's a number of other issues you need to consider.  Although doing it for your moral beliefs is nice in theory, it can be fraught with practical issues.

Nice to see you here BTW, frostschutz - IMO, you're one of the most talented coders in the community, have a good technical expertise in the field and generally very helpful too.