The current-generation BMW M5, internally called F90, has been around since 2017 and recently underwent a refresh. The even more powerful 2022 M5 CS entered production in March. So what's next for the powerful midsize luxury sedan? Well, BMW is definitely cooking up something, but the question is what?

Our spy photographers have just caught a strange-looking M5 test mule that boasts several modifications. Notice the rear track is wider than that of the standard M5 and M5 CS. The fenders have been extended and even the tires are wider. The M5 has 275-sized tires while the M5 CS has 285s. This prototype has a pair of 295s mounted.

Aside from the rear end, there are no visible changes elsewhere. So, what could we be looking at? An even more powerful version slotted above the CS? Doubtful. If it were, there'd very likely be additional changes throughout the body, most notably the front end. A more likely guess is that BMW M is working on a new special edition that will arrive for the high-performance brand's 50th anniversary next year. But again, this is purely speculation on our part.

Under the hood, the 4.4-liter twin-turbo S63 V8 remains, though we don't know whether it's the slightly more powerful version used in the CS, which has 627 horsepower over the standard M5's 617 hp.

The M5 CS also makes greater use of lightweight carbon fiber components. BMW M's half-century celebration next year is bound to be exciting. Not long ago, we caught the German automaker testing what appears to be a fully electrified version of the M2. Technically, it's possible because the M2 rides on a shortened version of the CLAR platform that underpins the i4.

We'd honestly love nothing more than to see BMW M debut more than one unique model to commemorate a very special occasion. It'll be interesting to see whether this mysterious M5 mule turns out to be something.