As others have mentioned, forking WordPress won't help. Writing abstraction layers might help a little (I've done that myself a few times) but instead, if you use WordPress, I'd encourage you to get involved with improving it directly. It's a slow process (especially as WordPress still supports PHP 5.2) but there is progress.
If you know PHP then there's loads of issues you could get involved with to help move it forward. Here's just 4 that could do with some attention:
If you know PHP then there's loads of issues you could get involved with to help move it forward. Here's just 4 that could do with some attention:
* PSR-0 autoloader http://core.trac.wordpress.org/ticket/21300
* Use a Symfony style request object instead of globals http://core.trac.wordpress.org/ticket/22325
* Use PDO for database access http://core.trac.wordpress.org/ticket/21663
* PSR compatibility http://core.trac.wordpress.org/ticket/23357