proportional representation doesn't require a constitutional change. a unicameral legislature that appoints and can remove the executive certainly would. 1/

however, re impeachment, as you suggest, a multiparty system brings the legislature closer to the founders' imagined legislative independence via "dilution of factions". 2/

in a well-sorted two-party system, Presidential impeachment almost impossibly broken. the legislature will usually be near 50/50, 50% perceive their own fortunes tied to the President's, can't get 2/3rds to impeach absent extraordinary unpopularity. 3/

with multiple parties, that no longer holds. whenever the President's party holds less than 1/3 of the Senate, much of the Senate can serve relatively independently as jury. 4/

(it also avoids the opposite bias: in a 2 party system, kneecapping the other party necessarily helps your own. in a multiparty system, members outside the President's party don't necessarily gain advantage from kneecapping the President's party.) /fin