I think it is very possible that Mueller will not find any direct criminal wrongdoing by Trump. His associates, yes obviously. Trump, perhaps not. Ever since Reagan-Iran Contra underlings have been very cognizant of providing their bosses plausible deniability. Treason is almost impossible to prove criminally.
Colluding with the Russians, getting peed on by Russian prostitutes, and dubious financial ties with Russian oligarchs may step over the threshold of ethical violations of the public trust. But Mueller is not investigating ethics.
I think there are political and ethical questions here that are just as important, if not more so, than the criminal question. Why was the Kremlin so keen on having Dump win?
Why did Trump spend so much effort denying the Kremlin was trying to undermine American democracy?
Why did Trump beg the Kremlin to hack his opponent - a fellow American from a different political party?
Why was Dump's son in law meeting with Russian operatives to dig up dirt on Clinton?
These actions may all be at the blurry boundary between unethical behavior, and criminal acts. But Mueller is strictly charged with looking into violations of federal law.