It has to be for the people who are making the case to leave the EU explain what they want instead. It would be totally unfair to both sides if the "remain" camp were to define the "Leave" side's position for them.
There are a multitude of possible options - the Norwegian/EFTA model, the Swiss model, the Word Trade Organisation option, or a bespoke deal negotiated particularly for Britain - and in the latter option you have to indicate what you will negotiate for. (It took several years for Greenland to negotiate leaving terms when they were the one previous area ever to leave the EU, and that negotiation was much simpler than setting up a bespoke deal for the UK would be.)
There is a degree of uncertainty about what "Remain" looks like too but by the time the referendum is called we should have a pretty clear idea.
Indeed one of David Cameron's main negotiating demands is to ensure that if Britain remains part of the EU the terms cannot be shifted against us (either by adopting financial regulations which sabotage our financial services industry or through the Eurozone using Qualified Majority Voting to push through measures which ignore the interests of the non-Euro countries.)
This is a valid concern for both sides, but at least DC is clearly aware of the problem and has a strategy to deal with it
Some "Leave" campaigners such as Dan Hannan are also aware of the problem but I have yet to see any sign of those campaigning for Brexit reaching a consensus on what they are arguing for. They will lose unless they do (which may, of course, concentrate minds.)
Assuming that the negotiation with other EU countries does produce a reasonably clear picture of what is on offer in the event of a "Remain" vote, then it will be a necessary condition for most rational people to even consider a "Leave" vote that the "Leave" campaign must spell out what sort of relationships they want Britain to have with the EU and the rest of the world so they can make the case for that to be a better deal than "remain" is offering.