What would it have taken for the Greens to reach 5%? What if Bernie Sanders didn't endorse anyone and some prominent Democrats defected to the Green Party?
It would take a pretty bad scandal to cause enough Berniecrat supporters to defect to the Green Party (considering that a larger portion of them would just write in Bernie's name, and some would vote for Trump or Johnson or another random candidate rather than Stein). Imagine if the email leaks included explicit demands from Hillary to DWS where she literally commands her to start fixing the process against Bernie and it came out on the first day of the convention causing like a massive walkout. That would give Stein 5%
I can't think of any Democrat more prominent than, say, Dennis Kucinich, who would actually leave the Democrats to support the Green Party instead - there's just no logical reason for any Dem. offiiceholder to do so.
1. A Gary Johnson that didn't get high on the campaign trail and stopped making dumb gaffes (and takes the campaign seriously enough to learn where Aleppo is)
2. A significantly worse scandal happens to Trump in late October. Either he's caught on tape using the N word, or one of his rape accusers isn't too scared to come forward in person and the trial actually starts going somewhere.
Definitely not in 2016, considering the above incidents require personal scandals to occur which wouldn't necessarily translate to a loss of support in downballot races. Also neither party was particularly well-positioned to benefit from any surge in support their presidential candidate might have. Both parties have minimal organizational strength and almost all of their down-ballot candidates are just random nobodies with nonexistent name recognition.
If the G/L parties poll high enough to secure federal funding and build up a bona fide party campaign infrastructure, it's entirely possible we'd see random celebrities running for statewide office in the following years - like how Jesse "The Body" "The Mind" Ventura used the Reform party's ballot line to get elected governor of Minnesota after Perot had twice polled above the federal funding threshold nationwide. Besides lucky wins like that I only see the Greens and Libertarians winning a few extra seats in state legislatures around the country, maybe with some random cities building enough of a party campaign apparatus that they're able to win city council seats and random mayor elections here and there.
Any major/consistent presence in statewide and/or federal office though would require sustained long-term efforts to push the parties into the mainstream - it would take multiple decades and would probably end up requiring widespread implementation of general election runoff voting (which would inevitably be passed as the parties grew big enough to provide spoiler effects across the country), if not another more proportional voting system. At that point the resulting political dynamics would be so extremely different from the real world that any answer would only be complete speculation.
