Please be advised I’m still a student and have spend some time working with NCA’s.  So this represents some “rough pointers” rather than anything definitive! ;o)

I'm aiming quite broad here as the way I see it the "specification" could be a high level description, or you could be looking to get down into the weeds and be looking for a full and exact description. (Right down to hardware/software used/ number of concurrent users etc). I'm not by any stretch of the imagination an systems expert but I suspect there's all sorts of Hardware/Software/performance issues to consider.  

In addition to Rob’s suggestions.

I assume you're already fitting this into some from of project management framework? Do the impacts of this have to be included?(Testing/implementation/training/handover/continued support/disposal requirements?)

How does it fit within the organisations IS/IM/IT strategies and will this add any constraints or requirements?   Are there any other current or expected programs/projects that may impact?

What is the driver for this change, what factors are "must haves" what are "nice to haves" Are there any opportunities to combine with other projects/programmes? 

Why not take a systems theory approach with Antony’s Triangle to help break it down?  Consider each aspect (input, process, output, feedback, etc) at Operational, Tactical and Strategic levels.  Then additionally the level of built in flexibility to change and integration into existing and possible future systems.

a) What are the inputs going to be, by whom
b) What processes will it need to perform, on what basis
c) What outputs will it need to provide. (Financial accounting data and management information).

For instance.
Financial accounting
What are the accounting policies that the module will have to implement and how much flexibility will they need for the future. – Asset life, deprecation method and expected residual life. Leased item data.
Are the assets ever going to be revalued? (why and how, will that need to be recorded as well).
What level of componentisation will there be? (Requiring linking between Parent and Child assets).
How will corrections be accounted for and how will the system manage it? (Say if something’s been entered with incorrect cost/life)

What audit trail is required? for (all of?) the above!

Will the assets be "enhanced" later, how will this be accounted for and how will the system implement this?
Group accounting implications?

Management information
Allocation of Deprecation, Remaining life’s on assets, Capital expenditure reports, NBV.  Level of granularity. (Product; Profit/cost centres; SBU, Division, Company).

You could then want to moderate that by how large the expected cost/benefits are (How intensive are your use of NCA's, if you've been audited have the auditors made any comments, etc).

Also what level of stakeholder involvement have you got (and could culturally expect in your business).  Getting the opinions of the users (both operators currently inputting the data and managers using the information) could be a double edged (maybe increased “buy in” and expertise, maybe increased resistance to change if they’re already suffering from Change fatigue) sword but might be worth considering.

 Then consider some P3 Management control / risk management elemets. (Access control, security, backup data, transaction logging/audit trail, etc).

And how about looking at what’s provided by dedicated packages on the market, that may provide further ideas?

Sorry it's a bit of a flow of thoughts and not well structured but hope that provides you with a suitable springboard of ideas. :os ;o)

JP