POD: Ney doesn't recall the Ist Corp during the battle of Ligny (june 16th). The prussian defeat is greater than IOTL, and without a Napoleon who curses Ney for the recall of the Ist Corp, Grouchy doesn't fear to break the ordered pursuit of the Prussians, to rescue the Emperor at Waterloo when he heard the battle.
This isn't a bad idea, but the biggest problem with it is that too big a French victory at Ligny almost certainly means there's no Waterloo. Wellington only firmly decided to fight at Waterloo after visiting the Prussians on the evening of the 15th and seeing for himself the condition of the Prussian army and extracting a promise from Blucher that the Prussians were still capable of fighting and would continue to support the British. If the Prussians are not just defeated but effectively destroyed so they are incapable of further combat then Wellington would almost certainly avoid battle by retreating north of Brussels and gathering up his outlying detachments and waiting until the Austrians or Russians get close enough to force Napoleon to divert forces.
This goes double incidentally if Blucher is killed or captured during alt-Ligny (he almost was OTL - he was actually ridden over by French cavalry who didn't recognise him) and command falls to Gneisenau who had a serious dislike and distrust for the British in general and Wellington in particular.
A more vigorous pursuit by Grouchy is also a possibility, but runs into the problem that the weather on the 17th was absolutely foul, and it would be difficult to overhaul the Prussians on roads that had been thoroughly churned up by the Prussian retreat (not to mention the usual difficulties of fighting a black powder battle in a thunderstorm). The end result is probably just that the battle of Wavre gets fought a few hours earlier, with a similar result.
One possible alternative to help Napoleon. The force he sent with Grouchy was both too large (to keep in contact with the Prussian rearguard and confirm which direction they were moving in) and too small (to defeat the Prussians if they stood and fought, or prevent them linking up with Wellington). Instead of sending Grouchy off with two infantry and two cavalry corps, send off just one cavalry corps with all the available horse artillery. This will stand a better chance of staying in contact with the Prussians and harrassing them enough to disrupt any concentration, while giving Napoleon an extra 30,000 or so infantry and 5,000 or so cavalry for Waterloo, adjusting his position from a slight numerical advantage (72,000 vs. 69,000) to a decisive one (100,000+ vs. 69,000). If he can't win in these circumstances then he really should retire.