Take a look - a 2' gauge Garrett - would be a great On30 model that might even find a following here in the States: http://www.railpictures.net/viewphoto.php?id=500458&nseq=3
I have one of these - a Backwoods Miniatures import. It is my showcase locomotive! It was featured in the On30 Annual a couple of years ago.
Beautiful machine.
Would gladly get another if Bachmann would just do one.
The PBR G-42, an actual 30 inch gauge Garratt would make another great model.
I would love any small or smallish Garratt. The original Garratt K1, would be my first choice, others are the Puffing Billy G 42 and the SAR NG16 posted by ebtnut. All are well known and preserved, K1 at the Welsh Highland (should be in Australia where it operated) G42 at Puffing Billy Australia and NG 16s at various places including in South Africa and the Welsh Highland. There is one being restored at Puffing Billy.
Much as I would like one I can't see Bachmann doing it - there is no guarantee that they will ever produce another On30 locomotive, and if they do it is likely to be a prototype from an American maker.
Hamish