There are obviously more than a few options and opinions about possible long lenses while waiting for the Fuji 100-400.
You're obviously going to have to settle for manual focus, but it is workable. I'd suggest being very careful in buying film era lenses. Older telephotos aren't as good as their modern counterparts. They often generate PF and CAs on high contrast edges and their mimimum focusing distance tends to be quite long. My own recommendation is for the Sigma Apo Tele Macro 400/5.6. They are a brilliant lens for the money. See the Photozone report (in Canon) where they say it's as good as the 400/5.6 L lens. They're not small - weighing about 1.4 kg. They're well made, apochromatic, good wide open, better at f8, internal focus, cause no PF or CAs, and are very close focusing for a 400mm. They focus to within two metres and a magnification of 1:3. Their only disavantage is that they don't seem to deliver good results with teleconverters. They come up occasionally on e-Bay and you'd need one in Pentax or Nikon mount to get an aperture ring for use on an XT1. About US$450-500 in Nikon mount, more in Pentax. They have the words 'Apo Tele Macro' on them and a 77mm filter ring. Don't confuse them with the earlier Sigma Apo 400mm f5.6 general purpose telephoto lens with a 72mm filter which were quite cheap and not in same league.
A smaller lighter option available for half the price is the Tokina ATX 400/5.6. Not as good as the Sigma ATM, but very portable.
If you're feeling like spending a much bigger budget, you could of course look at the Canikon ED alternatives. I personally wouldn't buy the non-ED versions - they may be solidly made, more easily found and cheaper, but you're back in PF/CA territory.
The LX100 has a 13mm X 17mm MFT sensor. I still have the first Canon G1X as my 'grab and go' or 'leave in the car' camera. It has a 14mm X 18.7mm 1.5" sensor and the size of the body and retracting zoom give some indication of the size an APSC fixed zoom camera might be if Fuji were to offer one as an X100Z. Obviously an APSC model would be bigger again. It could be a very good camera with a quiet leaf shutter, a decent EVF and a lens of a reasonable compromise in speed. I don't agree that the idea is a poor one. It would sell to a different market from the X100 series with the 23mm prime lens. It might in fact sell very well - as the LX100 has done for Panasonic.
My guess is that after all the comments made about the X30 at launch, there will be a 1" sensor in the next iteration of their X "compact".
Rod McD got a reaction from Curiojo in Lens Advice Please
Rod McD got a reaction from FoToEdge in Fuji X100Z