In his earlier translations Thanissaro translated papanca as "complication" which also seem to fit some contexts. And the word "diversification" or "multiplication" might apply as well (derived from the idea of "spreading" and "growth").
papañca: In doctrinal usage, it signifies the expansion, differentiation, 'diffuseness' or 'manifoldness' of the world; and it may also refer to the 'phenomenal world' in general, and to the mental attitude of 'worldliness'. In A. IV, 173, it is said: "As far as the field of sixfold sense-impression extends, so far reaches the world of diffuseness (or the phenomenal world; papañcassa gati); as far as the world of diffuseness extends, so far extends the field of sixfold sense-impression. Through the complete fading away and cessation of the field of sixfold sense-impression, there comes about the cessation and the coming-to-rest of the world of diffuseness (papañca-nirodho papañca-vupasamo)." The opposite term nippapañca is a name for Nibbāna (S. LIII), in the sense of 'freedom from samsaric diffuseness'.
Dhp. 254: "Mankind delights in the diffuseness of the world, the Perfect Ones are free from such diffuseness"(papañcābhiratā pajā, nippapañca tathāgatā).
The 8th of the 'thoughts of a great man' (mahā-purisa-vitakka; A. VIII, 30) has: "This Dhamma is for one who delights in non-diffuseness (the unworldly, Nibbāna); it is not for him who delights in worldliness (papañca)."
For the psychological sense of 'differentiation', see M. 18 (Madhupiṇḍika Sutta): "Whatever man conceives (vitakketi) that he differentiates (papañceti); and what he differentiates, by reason thereof ideas and considerations of differentiation (papañca-saññā-saṅkhā) arise in him."
In Thai translations, they always render it as "that which causes delay" (remaining longer in samsara) which is based on the commentarial gloss, but seems to lack much of the profundity of the original Pali term.
I have recently compared this notion of papanca to the infinitely complex chaotic systems which also start from a very simple formula but then become immensely diversified and multiplied in a recursive way.
In the compound papañca-saññā-sankhā the commentary glosses sankhā by koṭṭhāsa, “portion”, and Ñanamoli uses “calculation” because this term is related to counting, using numbers to represent reality. This would lead us to the following tentative translation: fractals of diversified perception