With this it's best to look at some existing specs and then making some extensive lists of your requirements and prioritise them. The big decision is how tough and how load capable you need it to be and is it a tour specific bike or are you thinking bikes for life that are good for unladen riding as well. The Thorn website, along with Peter White cycles, are good sources of info. I don't agree with all of it but it's detailed enough to let you understand the underlying arguments. Reading about Vernons Chimera is a good reference it is a well thought out bike. My new tourer/allrounder should be ready soon, frame is being finished off as we speak. These are my choices and reasons they may not be to everyones taste:-
frame - a custom Burls titanium frame with the geometry a mix of the Surly LHT and the Thorn Mercury. In terms of versatility I had the frame done with Paragon Rocker dropouts, disc brake only and cable routing for derailleurs or a Rohloff. This allows chain tension via moveable dropouts, the moveable parts are replaceable inserts so you can choose derailluer/non-derailleur on the driveside and rohloff or normal on the non driveside. They put the disc caliper on the chainstay so, unlike sliding dropouts, you can have normal rack/guard mounts. The clearances are up to about 700x60(29x2.2") though it will be run with 700x40 in general. The geometry specs a longer TT for use with flat bars(actually Jones Loop Bars) and the tubing is oversized to take the strain, at 29er level. The chainstays are longer 445-462mm depending on dropout position as I wanted stability and comfort over 'nippy handling'. The frame has 3 bottle cage mounts and a long head tube to allow me to get a more upright position.
forks - I'm waiting on a set of the new Surly LHT disc forks, I prefer them aesthetically to other steel disc offerings and they have low rider mounts. Though the overall frame and fork geometry is towards rear loading and with unladen riding in mind as well. Some traditional tourers will have quite short trail numbers for the front end to improve handling under load. My trail numbers aren't that short and should work with light front loading and unladen riding. But I have considered the possibility of different forks, maybe providing better laden front handling, if there is ever the need.
parts - disc only rohloff rear and SON Delux dynamo front wheels married to Supernova front and rear lights. Big mudguards that can accomodate up to about a 700x45 tyre, I like more clearance between tyre and guard than some. Avid BB7 MTB mechanical disc brakes with Avid Speedial 7 levers and Goodridge compressionless cable outers. The Loop H bars offering multiple hand positions and taking the standard Rohloff twist shifter. Shimano square taper BB and crankset with Thorn steel SS specific steel reversible chainring and outer bashguard. Thomson Elite seatpost, Brooks saddle and the other frame fittings are Hope, stem, seatpost clamp, spacers, headset, etc.
When deciding on this spec I probably considered and researched everything you will look at. I decided against belt drive and Alfine 11 as I felt they were both too new and teething problems are still appearing. Also I have a fair amount of experience of hub gears with chain drive and I didn't feel I had a need to move from that to belt. The 700c decision was based on aesthetic preference, it's a big frame(615mm ETT) and knowing that I wasn't planning any severe condition touring. I also have my 700c wheels built at Wheelcraft in Scotland and I trust Big Als wheelbuilding skills, I gave him my size and intended use and he built me wheels he felt would work. If I'm really honest the rohloff over A11 decision included the fact that you get QR with the rohloff and also the option of a red hub.
If you search around here, CTC, YACF and MTBR you'll find lots of threads about tourer builds covering every consideration...I know I've read a fair proportion of them
