A lot has happen in a few months, but personally I would not mind seeing an OOn3 loco on 12mm track what with both 009 becoming popular and Hornby now producing their TT system I wonder if it might be a good time to think about this.
For may sins I would suggest an Industrial 4wheel deisel; based on those of the Bord na Mona or if you want to the various Donegal and West Clare Railcars as for locos why not include the West Clare Bo-Bo deisels, Ok not everyones favorite and it may not be suitable, but If I knew it was going to be done then I would order three of them.
The thing is for 00n3 you need a reliable motot bogie and if the design can be used for other tt scale models, us OOn3 modellers can benifit so be it.
As for OOn3 steam locos as i see it you have three Irish size locos, tramway locos such as the Cork and Muskerry them middle ground Tralee and Dingle/ Cavan & Leitrim, then the larger stuff of West Clare, Donegal, Swilly and also the Ballymena and Ballycastle lines.
What I didn't include in this are two other scale gauges:-
1st HOm meter gauge French and German lines.
2nd HOm or what ever the Americans currently call it HO 3ft gauge.
I am not sure just how big these markets really are but it might be worth taking a look to find out and see if there is any commanality that could be involved.
As for developing N gauge most 009 modellers crave chassis's for their kits, mainly steam 0-4-0 or an 0-6-0 would be a good thing if you are looking at numbers.
Just to rap this all up, I don't know if any of the above is commercial viable or not since I am not in the model manufacturing game, but there again that my two pennies worth.
Regards
Colin Rainsbury