Yes, because it is bigger, has a higher population and actualy also a pretty decent economomical position. Wallonia will be influential, but will not be able to dominate, like it did in Belgium. The north must take Wallonia into account with the decission making proces. It cannot simply ignore it. But Wallonia will not be able to completely dominate the country. It will not be able to force French upon the country for example.
Wallonia won't force French in the country and thus it won't expand as it did in Belgium but if the United Kingdom is to survive, they will need to make quite a few concessions. After all, it will be a rich, influential, demographically dynamic and linguistically and religiously homogenous region (during the XIX/XX centuries) so independance or (re)joining France will be tempting to some.
Otl, in modern day Netherlands, there is roughly 29% french speakers. It seems likely that a sucessful language policy would promote french in the North and Dutch in the South as a second or third language and that number would be a lot higher. It is likely that, with Wallonia at the front, minority/regional languages in the low countries in general will get more recognition sooner.
Simply put, there will be Dutch governments with Walloon ministers, possibly even a couple Walloon Prime-Ministers, but they will be a minority.
That seems very likely.
walloons would be a tiny minority of 3million in a country of 26+ million population
brussels would likely become flemish again over time and i doubt the walloons would have much say in this country over time, they would not have the benefits they have in belgium that is for sure.
personally i suspect if flanders joins the north then the walloons wil either try to join france/independence or feel more and more dutch and flemish encroachment over time, something we see now arround brussels the other way arround
I'm not that familiar with Belgium demographic history, but 3million seems kinda low. According to Eurostat, there is 3.6million walloons (as in inhabitants of Wallonia) but 4.5 million French speakers in Belgium. The number of people that speaks French exclusively will be lower, certainly, but these that speaks two languages (or more) will be higher. French is still an incredibly prestigious and influential language, in the 19th century. That Wallonia is not so politically dominant in the United Kingdom that it was iotl in Belgium means that French will encounter a slower expansion but it will still expand.
Parma's point is very good, I think.