The really interesting issue here is that Flynn bothered to lie at all about these contacts with Kislyak. And the $64,000 question is, why did he lie? It seems unlikely that he was worried about the Logan Act, which bars private citizens from engaging in international diplomacy.
One possibility is that Flynn lied because he was trying to hide a longer course of contacts between the Trump campaign and Russia. An earlier guilty plea elicited by the Mueller team showed that Russia was trying to make contact with the campaign. The more contacts Mueller can show, the closer he is to a narrative that shows conspiratorial cooperation between Russia and Trump.
Flynn’s specific plea makes it harder for Trump to fire Mueller or pardon Flynn. The more Russia information emerges, the more any act of firing or pardon would look like obstruction of justice by the president. For now, Mueller’s investigation seems very likely to continue.