This Saturday, NI21 will hold its first party political conference at the Europa Hotel. Delegates will be asked to endorse a constitution and formally elect Basil McCrea and John McCallister as leader and deputy.
A few other appointments will be confirmed, including Tina McKenzie as chair and David Rose (former deputy leader of the PUP) as secretary, with five others elected from among the delegates to make up a nine-strong executive committee.
At that point, NI21 will become a political party, rather than just a movement, or the 'new voice of fresh politics here'.
The new party will face enormous challenges: building up constituency associations, attracting funds, setting out policies, fighting for media coverage with the other parties, campaigning and making itself relevant.
Most important of all, it will need to move beyond the rhetoric of presenting itself as a new vehicle and prove that it has correctly identified a significant gap in the electoral market.
While it is true that increasing numbers of people seem to be switched off by what the established parties are offering, it is equally true that the levels of cynicism and disengagement are so high (the latest Belfast Telegraph/LucidTalk poll indicates that almost 50% say they won't be voting at forthcoming elections) that NI21 may be knocked down by a 'plague on all your houses' syndrome.
NI21 was launched in June and, to be honest, not much seems to have happened since then. It was embarrassed by stories about polygamy, talks with the local Conservatives and beauty contests, giving the impression that it was lightweight and frothy.
It has had no big hits in terms of outflanking, or damaging, the other parties and it hasn't had any defections from either the UUP or Alliance; suggesting that potential supporters there have yet to make up their minds about the likely success of the new party.
It's worth bearing in mind that an awful lot of parties and groups have come and gone since 1970: indeed Wikipedia lists almost 70 now-inactive parties.
The United Kingdom Unionist Party, Women's Coalition and NI Unionist Party (all of which had MLAs in the first Assembly) have folded.
The Ulster Democratic Party, which took part in the Good Friday Agreement negotiations, has folded.
The PUP, founded in 1979, has just two councillors; the Conservatives, founded in 1989, have just one councillor (who defected to them); and the TUV, founded in 2007 (and which got 66,000 votes in the 2009 Euro election), has just one MLA and six councillors.
NI21 claims to have 'between 150 and 160 fully paid-up members' and is expecting about 250 members and potential members at the conference.
It has branches established at both Queen's University (around 30) and Jordanstown (around 15) and the nucleus of a handful of associations.
That's a good start in a short time. But what it lacks – and this will be the make-or-break factor for it – is a specific, easily understood identity and role.
It's not yet clear what it will be offering as a strategy for breaking through the ongoing political stalemate and general disillusion; and nor is it clear what its unique selling and branding points will be.
McCrea and McCallister are good performers and obviously comfortable with radio, television and live audiences.
What they must now do is convince both the media and the electorate that NI21 is about much more than them.
They need a strong team of spokespeople and candidates in place very soon and they need those people to be making an impact in their own right and in their own constituencies.
It isn't a given that either man will retain their Assembly seats in 2016, so much depends on how the party performs in the European election next May.
There was speculation that NI21 chair Tina McKenzie (who acquitted herself very well in the wake of recent allegations about her father's IRA and criminal past) would be their Euro candidate, but it now seems pretty certain that it will be John McCallister.
That will be a crucial test for the party: and it's not over-egging the pudding to say that if they fail it, then it will kill off their ambitions in one fell swoop.
In other words, they need to demonstrate that they can pick off votes from the UUP and Alliance, as well as attracting back lapsed voters and winning over first-time voters.
There is also a big challenge for the members and supporters who come to the conference, because it's not enough for them to merely nod in agreement and clap on cue.
Are they willing to knock doors? Are they willing to raise their heads and voices in their own areas? Are they willing to 'out' themselves to friends, neighbours and work colleagues as members of NI21?
A political party must be built on something more than just two high-profile figures: credible, lasting success will depend on the foot-soldiers and supporters rowing in behind them and taking their message to local doorsteps and regional newspapers.
On Saturday, NI21 will finally enter Field of Dreams territory: they have built it and now they must hope people will come in sufficient numbers to make it a key player in local politics.