I'm not Belgian so what would I know but my guess would be that RTBF would host it, most presumably in Brussels.
When they hosted in 1987, the Walloon broadcaster hosted and the Flemish one picked the representant, just as normal, but I can imagine RTBF/VRT agreeing that RTBF would also pick the representant if they host, with it being VRT's turn in 2026 again.
Alternatively, RTBF/VRT will collaborate and host Eurovision but what I hear from my Flemish friends, I feel like that wouldn't sit right with lots of people on both sides of the language border. I guess it's safe to assume that if Belgium wins in 2024, it won't be Antwerp 2025 lol