  The observation of heavy-ion-like behaviour in pp collisions at the LHC
suggests that more physics mechanisms are at play than traditionally assumed.
The introduction e.g. of quark-gluon plasma or colour rope formation can
describe several of the observations, but as of yet there is no established
paradigm. In this article we study a few possible modifications to the Pythia
event generator, which describes a wealth of data but fails for a number of
recent observations. Firstly, we present a new model for generating the
transverse momentum of hadrons during the string fragmentation process,
inspired by thermodynamics, where heavier hadrons naturally are suppressed in
rate but obtain a higher average transverse momentum. Secondly, close-packing
of strings is taken into account by making the temperature or string tension
environment-dependent. Thirdly, a simple model for hadron rescattering is
added. The effect of these modifications is studied, individually and taken
together, and compared with data mainly from the LHC. While some improvements
can be noted, it turns out to be nontrivial to obtain effects as big as
required, and further work is called for.
