I originally wondered whether the Mahayana shunyata (emptiness) is same as the Theravada sankhara (conditioned and compounded phenomena). The problem here is that Mahayana shunyata says even Nibbana is empty, but Theravada's sankhara does not include Nibbana. So, this does not match.
But after a lot of discussion here, I find that the Mahayana shunyata (emptiness) could be equivalent to the Theravada papanca (objectification-classification or reification), as found in MN 18 and Sutta Nipata 4.14. And Sutta Nipata 4.14 states that the root of papanca is "I am the thinker".
I could say that all papanca is empty of essence or substance.
How somebody (who is not an ariya) imagines Nibbana to be, is the papanca of it in his mind. In that sense, the papanca of Nibbana is empty of essence or substance.
How somebody (who is not an ariya) imagines a chair to be, is the papanca of it in his mind. In that sense, the papanca of a chair is empty of essence or substance.
Even the papanca of papanca itself is empty of essence or substance. This corresponds to Mahayana shunyata's emptiness of emptiness.
So, does it make sense to say that the Mahayana shunyata is same as the Theravada papanca?
Thanissaro Bhikkhu's explanation of papañca in MN 18:
Translating papañca: As one writer has noted, the word papañca has had a wide variety of meanings in Indian thought, with only one constant: in Buddhist philosophical discourse it carries negative connotations, usually of falsification and distortion. The word itself is derived from a root that means diffuseness, spreading, proliferating. The Pali Commentaries define papañca as covering three types of thought: craving, conceit, and views. They also note that it functions to slow the mind down in its escape from samsara. Because its categories begin with the objectifying thought, "I am the thinker," I have chosen to render the word as "objectification," although some of the following alternatives might be acceptable as well: self-reflexive thinking, reification, proliferation, complication, elaboration, distortion. The word offers some interesting parallels to the postmodern notion of logocentric thinking, but it's important to note that the Buddha's program of deconstructing this process differs sharply from that of postmodern thought.
From Sutta Nipata 4.14:
"I ask the kinsman of the Sun, the great seer,
about seclusion & the state of peace.
Seeing in what way is a monk unbound,
clinging to nothing in the world?"
"He should put an entire stop
to the root of objectification-classifications (papañca):
'I am the thinker.'
On objectification-classifications and their role in leading to conflict, see Sn 4.11 and the introduction to MN 18. The perception, "I am the thinker" lies at the root of these classifications in that it reads into the immediate present a set of distinctions — I/not-I; being/not-being; thinker/thought; identity/non-identity — that then can proliferate into mental and physical conflict. The conceit inherent in this perception thus forms a fetter on the mind. To become unbound, one must learn to examine these distinctions — which we all take for granted — to see that they are simply assumptions that are not inherent in experience, and that we would be better off to be able to drop them.