Several mind-map tools offer the ability to cross-link, or add relationships that are not hierarchical, and these are often very important relationships.
For example, in the MM below, the green arrow is a cross-link from Another topic to common topic.
I can think of ways to do this in PlantUML, with the simplest being that if a text for an item is the same, it's linked with an arrow the second (or subsequent) time it has been used. However, this may not give all the results one might expect (it's implicit based on ordering). Another way could be to use a linking syntax, like is done with Notes nodes in class diagrams, e.g.:
** A topic
*** "common" as c1
** "Another topic" as t2
t2 ..> c1
There's probably a better way... but the feature would be very useful to me.