Except for the waiting until the EU becomes the United States of Europe part. Our suggestions tend to focus on the present or near future, not remote-and-improbable-future.

Opinions are rather divided among the Francophones as to what the best solution is if Flanders should decide unilaterally to split off (if it doesn't, they certainly won't take the first step). Generally, many Walloons would hesitantly accept rattachism if that happened, and so would some Bruxellois. Fortunately there are enough sensible people in Brussels to realize that Brussels in France is utter suicide - Paris is notorious for hogging the limelight and centralizing everything, leaving even the biggest of the other cities of France in a status of second-rate city, and no way in hell will the UK or Germany ever accept having both seats of the EU parliament in France. My personal opinion is if Brussels ever ends up in France, it'll take about half a year for the European institutions to move to Luxembourg - unless the East-Europeans are strong enough by then to demand part of them move further east.
On the other hand you have some delusional Flemings claiming that "if we split up, Brussels will choose with its wallet and join Flanders", but I think they're rather overestimating the wealth of Flanders there, and taking their wishes for reality.

So there are only two real options for Brussels in that situation - "WalloBrux", a Belgium-minus-Flanders, or "Brussels DC". On the whole, I think WalloBrux would win out, as I don't think rattachism would carry the day in Wallonia. They'd have a geographical problem, though (Wallonia and Brussels aren't contiguous territory, they don't share a border), unless they get some Flemish territory somehow.

There are some advocating more original solutions, though. My dad is and remains convinced that Wallonia would split into several parts, joining respectively Germany (our tiny German-speaking minority), Luxembourg (the Ardennes), Flanders or Brussels (Walloon-Brabant) and France (the rest).

Yep.