At the risk of doing a "crash and burn", I am going to set out what I think is the most likely outcome of tomorrow's General Election in the UK.
It is quite clear that no one party will have an overall majority. I believe it is quite likely, almost certain, that no combination of two parties (apart from a Conservative/Labour coalition or "supply and confidence" arrangement) will command an overall majority, the shortfall being at least a dozen. The probably most likely outcome will be that the Conservatives will get a small plurality of seats over Labour. Exact numbers are difficult to assess but the likelihood is that the number of seats will be in the 40-50 range for SNP and 28-35 for the Liberal Democrats. UKIP, despite getting a larger percentage of the national vote that the Liberal Democrats, will end up with one or two seats (five at the most). The Greens and Plaid together will total fewer than the various Ulster Unionists. SDLP and Alliance (in Northern Ireland) will get a small handful of seats.
Cameron will attempt to form a minority government but will need both the LibDems and the Unionists for "confidence and supply" arrangements (C&S) to get a Queen's Speech past the Commons. Much will depend what proportion of "Orange book" LibDems are elected, but I would expect accommodations agreed with the Unionists by Cameron in exchange for their support would scupper a coalition or C&S. I doubt if it would get the agreement of the party in the House, let alone at the special party conference required under LibDem rules.
That leaves the Labour Party with an opportunity to negotiate arrangements with other parties. Here their stumbling block is that their leader, Ed Miliband, has ruled out any arrangement whatsoever between Labour and SNP, even a C&S. He has, however not ruled out arrangements with the LibDems but I think in the most likely outcome, the two would not form a majority coalition or have enough to pass a Queen's Speech on their own. Here is where it starts to get interesting.
A small but very influential wing of the SNP is adamantly opposed to the UK's nuclear submarine fleet which is based in Scotland. It will need replacing and decisions will have to be made in the next few years as the existing boats and missiles are getting towards the end of their planned life. Labour propose replacing the existing 4 submarines with 3 so that still one can be permanently at sea. The Liberal Democrats would only order 2 and abandon the permanent patrol. SNP want no replacement. Here I think is where some "interesting" discussions could take place. IF Labour and the Lib Dems agree a coalition, the Lib Dem team could start discussions with the SNP without Miliband breaking his campaign pledges. An option offered might well be to "kick into the long grass" the matter of replacement by agreeing a Royal Commission or inquiry to examine options and moving the current base from Scotland to one of the Royal Navy bases in England or Wales. A lot of Labour members would also be happy to abandon Trident and having to reduce the proposed 3 submarines to 2 and then leaving the replacement decision to later would appeal.
Labour Party anger at their losses to SNP will likely mean they are hostile to a formal coalition between their parties. A confidence and supply arrangement to support a Labour/Lib Dem coalition, with the other negotiations done at "arm's length" by the Lib Dems could well satisfy them. It gets "Ed into number 10" and potentially produces a more stable if minority government. Further C&S support is likely from the Labour and LibDems sister parties in Northern Ireland (SDLP and Alliance) and from Plaid and the Greens on specific bills.
If I am right in my assessment that no two parties would have a majority, but this scenario does not pan out, expect another election soon. This could be as early as July but could not be until next February or March. There are traditional reasons for this: the annual election register takes effect in February but is based on residence the previous September so is "fresh". Recent changes to allow late registration have reduced this impetus.
Another "window" is from early November which would be after the annual party conferences in September but before the winter weather and dark evenings. Certainly the Conservatives, either way, would like to delay an election until UKIP becomes the obvious busted flush they will be without PR. Ironically, a new examination of electoral reform, which would have boosted them as it did in the European elections, will be too late to save them. Labour will also be interested in having another election as late as possible so that they can rebuild their local party structures in Scotland - a big reason for their defeat will be the moribund state of many of their local campaign organizations because they did not have to work to get out the traditional Labour vote in safe seats. (It's the main reason Liberal Democrat Simon Hughes was first elected in a by-election in Southwark. Their problem is that this experience shows that a small local party structure is quite difficult for them to rebuild once a popular MP gets in place. I should be working for Simon rather than writing this so I might not be around until much later to respond.)