Most of these would be small, especially in a country like the US were people are used to having 2 options
And in the change I propposed people would still need at least 30% to win seats. I believe you'd end up with a few partiis with varying strength in regional or state party systems, like in the UK.
Even full-PR or MMP wouldn't require or even allow for 15-party coalitions, and if it happened you could just have a treshold. Turkey for ex is the extreme case with a 15% threshold, and in europe 3 or 5% are common, as are district-based systems.
Look at Mexico's system for ex, or regional UK ones, or Irelands (a form of preferential voting), Australia's, NZ's. All multiparty, none have 15-party coalitions.
Usually there are 2 dominant parties and a couple medium or smaller ones, or 3 main parties or at most 4 (with two being larger than the other 2), and coalitions change little in composition (but can change a lot in the relative size of each member party due to competition).
I doubt an antifa party would evee grow to be significant enough in the US. You'd likely have typical conservatives, paleos or nationalists, 1 or 2 moderate parties, progressives and as smaller parties greens and libertarians.
Whichever party wins the presidency would tend to be larger and more powerful too. Better to compare to France or Mexico in this case, instead of euros. A president changes everything because he has more bargaining power
SYRYZA will lose the next election btw. People hate them for being incompetent now. And they're greek, not american. Thete aren't a bunch of leftist currents in it because of the system, but because they're greek.
IIRC theyre changing their system to a bonus one like Italy's, which I think is shitty.
But there are plenty of options to find what's suitable for a country.
The US could easily live with the Irish or Strayan or Kiwi systems