In a fantasy multiparty system, which party would you join?

[IceAgeComing]

The system laid out in the OP wouldn't last for any appreciable time; you have too many parties that are basically the same thing (SDP/Feminists) or which have a base that is tiny to the point where they'd only get in to the legislature if America adopted a Dutch-style thresholdless List PR system or where the people who you've tapped as being in it wouldn't create their own party but run in one of the bigger ones.

An American multi-party system would be the sort to retain two broad church left/right parties: perhaps with the far left in a more openly socialist party and the Republicans would probably fragment in a similar way as well (in the past I'd have said that the populist element might have formed into its own party but now it makes more sense to consider that your "TRUE CONSERVATIVE!!!" elements (like Kasich or Cruz - different wings of the parties but broadly similar economically) might form a splitter party.  Additionally a multi-party system would probably lead to a much higher use of party whips and a great deal more party unity than is typical in American politics currently since the continual rebel would be chucked out, or would leave and join someone else instead.  That's why talk of being in the same party as someone but not "caucusing" with them is stupid in a multi-party system: in any voting system where you'd have six or seven parties the party that had the likes of Santorum and Casey in it would split really very quickly - and besides, what's the point of voting for a party if you don't know exactly who they are going to support in the legislature (again for this system to be viable you'd need to have list PR: and even in an Open List even when you vote only for candidates on the list that you like you have to support the entire list); especially if they'd end up divided.

A multi-party system probably would be very similar to now really: you'd have two big parties (we'll call them the Democrats and the Republicans for ease of this analogy) although the left of the Democrats probably would form a more openly Socialist party.  The Republicans would probably split as well tbh; although exactly what that split would be is more complicated (in the past I'd have said that you'd have had your populist-right Trump types form their own group although now it makes more sense to think of your TRUE CONSERVATIVE!! types splitting off with the Republicans becoming this odd vaguely populist-right party.  The Libertarians probably would get in to Congress from the current minor parties (as might the Greens but whatever the new Social Democratic party is might take their support from them): you'd probably get someone trying to form a vague party of the centre supporting the status quo (liberal economics: not rocking the boat on social issues either way) but those sort of parties have fallen back globally in recent years.  You'd still basically have a two bloc system; its just that the factions within those blocs are separate parties rather than loose factions.