This artwork by Dave Filoni has apparently fueled Rumors that Rex may be joining Ahsoka in
The Mandalorian.
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It wouldn't be a big deal if this was simply Dave doing something that speculated "What if Rex and Ahsoka appeared in
The Mandalorian?" If he's that intent on including either or both characters in future episodes, though, the natural question is "Why?"
if either character had a reason to appear and be part of the story as a guest or cameo character, it might work or make sense. However, I fear they'd appear simply for fan service and to shoe horn them into the show in an attempt to have Ahsoka (and now Rex?) appear in as much SW media as possible.
Not knowing the timeline for
The Mandalorian, I'd also wonder if Ahsoka appearing in it would conflict or cause plot holes with the events and her actions in
Star Wars Ahsoka. Given how Disney's new SW canon has so many inconsistencies with itself, however, Disney/the SW story group probably won't care unless too many die hard fans press the matter. Maybe (big emphasis on maybe) then would they offer an unimpressive explanation or retcon in response.
For someone who might offer the counterpoint, "Gee, didn't C-3PO and R2-D2 appear in all kinds of SW media?" I'd argue that while that's true, George Lucas ultimately included them when and where it made sense most of the time (along with numerous scenes where they provide campy comic relief). Rarely were they shoehorned into something as a gratuitous attempt to include the characters for no other reason than the sake of including them.