My original plan for what became
the Battle of Velaine in my TL was pretty much this — Ligny with Ney able to join forces with Napoleon because Wellington wasn't there to keep him busy. Then I realized there was no real reason for it to happen at Ligny.
I can think of three possibilities for OTL's Ligny, if Ney does this:
1. Wellington comes to Blücher's rescue.
2. Blücher is defeated, falls back and joins forces with Wellington. A day or so later, they fight Napoleon and Ney together.
3. Blücher is killed or captured and his army taken out of the picture. Wellington fights Napoleon and Ney alone.
Of course, even if Napoleon defeats Blücher and Wellington, he still has the Austrians and Russians to deal with… or, more likely, to be dealt with by.