Relativistic quantum mechanics. The big problem is quantum gravity. Special relativity has already been reconciled with QM.
If you don't have the interest or ability to express an opinion in that matter, perhaps you would consider solving the "puzzle" described in the first two sentences of a research paper cited in this article: More than one reality exists, shocking study says
"The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them. In quantum mechanics, the objectivity of observations is not so clear, most dramatically exposed in Eugene Wigner’s eponymous thought experiment where two observers can experience fundamentally different realities."
Which trumps which: the Scientific Method or Quantum Mechanics?
Inquiring minds want to know what you think, Professor.