No, wait a second. Rereading what i have written i'd say that adding an extra [w] is way overpowered... it's probably more balanced this way
Lance Fighting [Feat]
Prerequisite: Proficiency with longspear, Mounted Combat
You can use a longspear one-handed while mounted. While charging a creature smaller than your mount you knock the target prone on a critical hit.
Using the longspear one-handed is about valuable as a weapon proficiency feat, and the part about the critical hit is similar to the surprise knockdown feat. The balancing factor should be that you have to be mounted, so it's not always available...
Also, i'm not sure about having "mounted combat" as a prerequisite. It makes totally sense, but i seem to remember that feats in
4E can't have other feats as prerequisite (eg. no feat chains).