Since there isn't really any great source port of Wolf3D, I was wondering if it were even a slight possibility that XL engine would ever expand to include Wolf3D?

NewWolf is cool and all, but it's very old, doesn't have mouselook at all, and hasn't been updated in forever.

I suppose if ArenaXL is ever made, then there would be all the infrastructure to load tile-based games like Wolf3D. Keep in mind that with four games at the moment, Lucius already has his plate full. Any additional game, if they happen at all, will have to wait until Blood, Daggerfall, Dark Forces and Outlaw are all 100% operational and as accurate in their behavior as possible.

In the meantime, I'd suggest you keep an eye out for ECWolf's development.

There is a great port of Wolf3D called Wolf4SDL. It's all you'll need in a Wolf3D. I mean you really can't expand on it all that much, and all the GL ports are very lame and mostly unstable but Wolf4SDL is great.