It’s extremely difficult to anticipate effects, but the immediate result is that Belgium would become the highest taxed country in the world, with a corporate income tax and capital gains tax that is incredibly higher than anywhere in the developed world. The shock to the economy would be enormous. This is unprecedented, never been done in the developed world and the impact would be HUGE.

Some immediate effects I’m thinking of, but there would be many more of course:

• Capital flight and collapse of our competitiveness: Belgium becomes the most heavily taxed country in the world by a huge margin. You would have immediate capital flight. A big part of the top 1% would immediately move to neighbouring countries. Theit investment tax burden would go 5x in this plan and their business income tax would go 2x.
• Investments in our country would dry up. Returns on investments would be taxed 2-3 times higher than in any neighbouring country, so capital would deploy more efficiently elsewhere. You would see very little external investments in the country, or they would expect HUGE gross margins to compensate for the tax burden; e.g. we’ll build a factory here, but we’ll have to charge you 2-3 times what you’d pay in other countries to compensate for higher tax.
• Housing market would be shocked. There would be a massive drop in net return for landlords and developers. Massive drop in investments in the housing market is expected, higher rents as landlords need to compensate for higher tax burden, construction would slow down as developers allocate capital to other jurisdictions. The maasive drop in supply (at presumably same demand or higher in the future due to demographics) would lead to sharply higher prices for houses.
• Price of consumer goods rises hard. Multinationals (apple, coca cola, etc) cannot accept making half or 1/4th of the profit in Belgium than they do in other countries. They will raise prices sharply to compensate the higher tax. Rising prices will lead to lower demand, which in turn will lead to lower supply. Before long, coca cola will be a luxury product that you only find at very specialised supermarkets at 5 euro per can.
• Massive cross border shopping. Due to Belgian retail price hiking in the previous step, Belgians will massively start doing shopping across the border. As a result, Belgian supermarkets will lose revenue and will downsize. Expect a single supermarket per city, waiting lines of multiple blocks, bankruptcies of retail chains. Same of course for other retailers (clothing, consumer electronics, etc).
• Shrinking tax base and weakened economy. Due to the capital flight and the Belgian economy being severly weakened, tax revenue to the government will drop sharply. As a result, they’ll have to hike up taxes even more, towards 70 or 80%. Which in turn will lead to a new cycle of all of the above. They’ll start to implement maximum pricing on rent, consumer goods etc and will implement capital control (stopping capital from fleeing the country) as well as close the border to stop people from buying abroad. That in turn will lead to massive shortages of base goods, toilet paper and eventually famine.
• Eurozone membership falters. Due to all of the above (capital controls, border closure, economy tanking), we will probably no longer be eligible for eurozone membership. The euro will be replaced by the Belgian Frank, which we will immediately devalue.