It wasn't that long ago that I cited an example of Mike Curtis and Joe Staton bringing another comic strip into Dick Tracy's universe (see: Dick Tracy's Dream). The sequence below ran July 12-14, 2016 and brought another long-running comic strip character into the Tracyverse.
I'm not sure if this appearance is somewhat elliptical because of copyright restrictions, but it certainly fits with the character.
Long-time comic strip readers will recognize the Phantom in his Mr. Walker persona. Although the figure is never clearly shown, his upturned collar, low-slung fedora, and dark glasses provide more than enough clues. And the wolf is, of course, Devil as Tracy almost directly says.
In a way, this addition is a twofer. Both the Phantom and Mandrake the Magician were created by Lee Falk, and share the same universe (having appeared in each other's strips). So if Dick Tracy and the Phantom know each other, then Mandrake is part of the Tracyverse now, too.
And as part of the same story arc, another cameo was brought back. Deathany Denobia of Bill Holbrook's On the Fast Track was already established in the Tracyverse (see: Dick Tracy on the Fast Track). She returned in an early August 2016 sequence to provide information about the sidekick of Abner Kadaver, a horror show host turned hitman.
One-off cameos can be fun. Subsequent appearances by them can be even more so.