This depends on what one thinks the 'Theory of Everything' looks like. If there is a steadily increasing ladder of more and more massive particles ad infinitum, then as Physicist Hagedorn showed in the late 1960's, the maximum achievable physical temperature in the universe may be as low as a few trillion degrees. What happens is that as the particle thermal energy increases, the energy goes into creating progressively more massive particles with low kinetic energy. Eventually, no matter how much energy you 'feed' into such a system, all that happens is that you generate more massive particles with a lower kinetic ( thermal) energy. In essence, this pair production of massive particles acts like a 'coolant' which regulates the physical temperature at a finite, and perhaps surprisingly low temperature.
If the so-called Standard Model is correct, then quarks are fundamental and do not sub-divide into still more massive particles. The thermal energy is then free to increase practically without limit until you eventually end up creating 'quantum black holes'. This happens at a temperature of 10^32 Kelvin; that's 1 followed by 32 zeros! Once you produce quantum black holes, you have reached what many theoreticians believe is the end of the road for physics as we know it. Here, spacetime itself dissolves into a witches brew of quantum worm holes, black holes, multi-dimensional super strings and twistors. Most theories of the early universe give this temperature as a true limiting temperature for physics in the universe.