It strikes me that this is a particularly American, or at least presidential system, question, in that you guys have a far less defined sense of who "candidates" are for a lot longer. The presidential election is nearly a year and a half away, but because of the primary process all the jostling is happenning already. The candidate "field" is far less defined in such a strongly individual focused system.
18 months out from an election in a parliamentary system, you generally already know who all the potential next heads of government are,. They are the leaders of the main parties and the possible "field" is well defined for the next government in terms of both leaders and other potential ministerial figures.
At all times, the realistic range of head of government candidates is known. And the field may often only two or three people. Such contenders nearly always already have a seat in parliament and exist as part of a party with all the philosophical detail and history that this brings. The ability of people to define themselves separately from their party is less, compared to a primary system like in the US where people from the same party very publically compete against each other. (some parliamentary systems like Canada and the UK have parties choose leaders by public vote or from amongst members, but those events are very strongly branded and they're still largely about the party)
Parties also have a permanent presence and sense of central control, in contrast to the Republican and Democratic Parties. Outside of election times, the Dems and GOP are something closer to a common brand label and fundraising organiser, in comparison to weighty between-terms institiutions with backroom figures, administrative functions, etc. In a presidential system there is not, notably, a figure who could be described as an "Opposition Leader" to the president (the house and senate leaders don't have the profile or backing, and don't usually end up running for president).
In terms of the question about newsworthiness, that's got easier answers in a parliamentary system. If there's a new and emerging party that rises up the polls, there's no question about whether that is worth covering, because it always will be. Whether it's Podemos, Syriza, UKIP, the Clive Palmer United Australia Party (a vehicle for a billionaire here), their emergence is necessarily a news event. Any figure who emerges as an independent force will likely only be of local interest if they run in a local district, or they will organise a party around themselves. Minor parties largely get attention based on their popularity or notoriety, and often have to have an electoral breakthrough before getting serious attention (to wit: the Canadian Greens still being excluded from televised debates), and microparties are pure novelty value in reporting.