While Mueller may have a new can of worms to pursue, it's also probable he wasn't just waiting for the 2018 election, he actually has to wait until the new Congress takes office.
Compare this to Ken Starr who looked under rock after rock and never found anything except suspicion, much of which was like the Benghazi and email obsessions, nothing worthy of criminal or even civil pursuit.
Mueller OTOH, has so much criminal material he may just have to stop because there is no end to the crimes. There are articles that Trump has been money laundering for the Russian mob for decades.
Salon: Investigative reporter Craig Unger: “Trump has had contacts with the Russian mafia for 35 years”
Author of “House of Trump, House of Putin” on the president’s long history with Russian oligarchs and murky money
I've not heard that book was debunked like several of the alt-right gems: Clinton Cash (pure BS) and John Fund's book on voter fraud (another pile of BS).