Hi Democracy Junkie,
From what I read on your link, the Westminster system is itself a parliamentary system, though I do not know much about how it works and how it is different from others. But what I can tell you is that I have played D3 for some years now, and noticed that your limitations from taking new measures would come from the political capital you have available to spend, and that in the end you are running for office against one other party, so it would be based in a bipartisanship (don’t know if that word exists, but I mean two parties running against each other, mostly like in the US Presidential system, where Democrats run against Republicans - in the US I believe there are indeed other parties, but those are irrelevant when compared to the big 2).
What I meant by multiparty democracy would be a system where you run against 3/4/5 other parties, and then control x delegates, while the other parties control y,z…In cases where you control less than 50% of delegates, you would have to negotiate measures with a portion of delegates from other parties. For example, you need to raise corporate tax, but you control only 40% of the house. Then you would have to ask other parties to go along. Most likely, right wing parties will not be for it as they are more pro-capital and less pro-unions, so you would have to turn to the left wing, more socialist. But, if you’re a centre party, turning to the left wing could make some people from the right side of your party switch to the right wing extremists, and so on… As you said, it would be cool to have this in future developments, but might require a lot of coding.
I can give you some examples of European Parliamentary Schemes:
UK - Queen is the Head of State. Elections are for the house of representatives. Some parties are: Conservatives (Boris Johnson), Brexit Party (Nigel Farage), Labour (Jeremy Corbyn), other smaller parties (Greens, liberals, etc)
Spain - King is the Head of State. Elections are for the parliament. Some parties: PSOE (Pedro Sanchez), PP (Pablo Casado), Ciudadanos (Albert Rivera), VOX (Santiago Abascal), Podemos (Pablo Iglesias).
France - President is the Head of State, but things are different. I think president is elected and then there are also elections for the parliament.
Italy - President is the Head of State and is elected. Some parties: Liga Nord (Umberto Bassi), Forza Italia (Silvio Berlusconi), M5S (Beppe Grillo), PD (Nicola Zangaretti), Italia Viva (former head of PD, Matteo Renzi)
Germany - don’t know exactly how it works but major parties are CDU (Angela Merkel, being substituted by AKK), SPD (Saskia Esken/Norbert Walter Borjans), AfD (Jorg Meuthen/Alexander Gauland), Grune (Simone Peter/Cem Ozdemir)
Portugal - President is Head of State, elected for 5 year mandate (can be supported by parties but are not representing them). Parties currently represented in the parliament (elections are separate from president’s) are PS (Socialists, António Costa), PSD (Social Democrats, Rui Rio), BE (Left-wing antisystem party, Catarina Martins), PCP (Communists, Jerónimo de Sousa - usually running in coalition with the Green party), PAN (pro-Animal and nature party, André Silva), Joacine Katar Moreira (independent, ran for parliament with LIVRE but left the party after the election), CDS-PP (Centre party, Francisco Rodrigues dos Santos - party is somewhat sibling of Merkel’s), IL (Liberal party, João Cotrim de Figueiredo), CHEGA (Right-wing extremists, André Ventura). About this system one interest thing that we found out in 2015 is that you don’t have to have the majority of votes to lead the government: back then, PSD ran for office in coalition with CDS-PP and had about 37% votes and claimed to have won the election. However, after the election, PS formed a broader coalition with BE and CDU and, with their approval in the parliament, they managed to form a government, controlling 50,75% voting in the parliament. They needed also the approval of the president, which was granted. That’s how interesting it is: the socialists had to make a coalition with extremist parties, ultimately having to negotiate part of the budgets with them so they could have them pass at the parliament.