Before Trump fired Comey, the FBI had opened a counterintelligence investigation against Trump. Comey likely knew this was coming, which is why he was unwilling to tell Congress that the president was not a target of any investigation, but merely a subject. It turns out that Trump became a target even before Comey was fired.
Muller has Trump's iCloud data, and likely every email and text sent by anyone in the White House. If it turns out that Trump knew about this before he fired Comey, he will be indicted for obstruction of justice and almost certainly convicted on that charge alone.
More ominously, if it turns out that Trump was in fact doing the Russian government's bidding (as seems increasingly more likely given his actions from the 2016 Republican convention to present, to the point that it is a near certainty among those who have been paying attention and who do not view the world through the blinders of "loyalty" to Trump), he will be indicted, prosecuted, and convicted of treason.
Examples of treason:
1. Changing the US's foreign policy platform in relation to intervention in Ukraine in exchange for something of value from the Russian government.
2. Conspiring with agents of the foreign government, directly or indirectly, to change US policy related to intervention in Syria in exchange for something of value from the Russian government.
3. Relieving or promising to relieve, or refusing to enforce or implement, sanctions of any kind, including in particular, those against Oleg Deripaska and those generally targeted by the Magnitsky Act, in exchange for something of value from the Russian government.
4. Offering, or promising to offer, any particular US citizen, or former US Citizen, including Bill Browder, to the Russian Government, in exchange for anything of value from the Russian government.
5. Receiving, conspiring to receive, or knowingly directly or indirectly benefitting from election assistance of any kind from the Russian government or any agent of group of agents from the Russian government; in exchange for anything of value to or from the Russian government; either on an isolated basis, or on an ongoing basis.