Wellington was very concerned about Brussels and Ghent - and it was one of his criticisms of Clausewitz's work that the latter only thought in terms of the decisive big battle. In my opinion, Wellington was right in that much of what Napoleon said/did supports the concept that this campaign was more political and army destruction.
What is interesting is that the same debate goes on today!
Regardless, from a simulation perspective, I am absolutely fascinated by a variety of scenarios. The Prussians decided to concentrate on the north side of the Meuse at the Sombreffe position with a move on Charleroi - something that was agree to with Wellington in May I believe. However, why not concentrate south of the Meuse and immediately go for the rear while keeping their communications with Leige.
One could argue that a triumphant entry into Brussels would not have mattered if all that Paris heard was of a Prussian army cutting across the northern frontier.
At this point, some tremendous research would need to be done to adequately understand the provisioning of armies that are potentially cutting each other off, and these very real concerns are one reason why reality was never quite as fluid as a game.
One thing I would suggest is to, regardless of start date, eliminate the early Prussian concentration. Battle of Ligny began in mid-afternoon - Prussian II and III Corps had marched all the previous day, many units past midnight, and were up again early to reach that battlefield. You remove 12 hours, and there is no battle of Ligny, and then you have a fairly wide open campaign with choices abound.