Oh, this is 'A' consultation process, i didn't realise.
Oh well, in that case.
Have separate subjective and objective areas to stop each group boring/driving away the others.
Have a decent music review section with some sort of interface that allows for a chart style ranking based on votes of members, maybe a drop box for samples of same. Most record labels will give you agreement on a waiver for this purpose if you provide the waiver and include a most 'favoured nation' clause.
Allow people to pay for the site and gain benefits from doing so, hosting space for pictures for example, larger message box etc.
Host tech docs and manuals for popular products and also for basic electronics engineering skills, soldering videos, resistors calculators, filter calculations, handy little applets that sort of thing.
Have a tech section where there's a weekly/monthly discussion topic where people can go out and do a little research before they enter the discussion.
Send out formal invites to respected designers to engage with the members, give them an area where they can discuss their products and or ideology. Pitch them that in the run up we'll create them a 'product history', an online chronology with products listed and pictured, something they can take value from they'll be more likely to take part. Once you've done one layout its a piece of piss to re-use, just have a document template for the data you need and any monkey can do it.
And sort out a 'pay for' trade area if you intend to be taken seriously, no one values anything they get for free. Match the cost to the benefits. Most good forums are run as business's if you want to be able to spend the time on content, or incentivise people to produce the good stuff for ZG then this is the only way it'll happen. Maybe use the dealer ad revenue to create a ZG credit for regular quality contributors that they can spend at selected dealers.
I guess it depends on how much time and effort you wish to invest.