Perhaps Leica should manufacture two distinct product ranges-one boasting traditional construction, limited upgradeability (at some stage a viable digital back will be made available), and an expansion of the "luxury product placement concept" (probably best met by emphasizing the "a la carte" offerings) , based on the existing M-range. Additional services could be developed ("certification" and overhaul of used M-cameras, training, lifestyle events (for new markets), and promotion of photographic standards- sort of an upmarket version of what LOMO have done. This would ensure the continued viabilty of the M-system going forward.
The second, more challenging concept would be to find a stable long-term range for the digital market, whose underlying technology is changing so rapidly. This Leica must resolve some of the current weaknesses (not insubstantial) inherent in rival digital product offerings.The Leica digital top-end offering should most definitely not be a me-too product - this has been made amply evident (commercially, at least) with their SLR range over the years. Not easy, but Leica are probably quite well placed to think "outside the box", especially as they would not really be weighed by the legacy issues challenging their competitors.
In fact, Leica's strengths have historically originated from it's optics, and the interoperability and upgradeability of it's system. Taken to a logical conclusion, this should mean that any new range should incorporate a cohesive imaging capture and delivery unit, able to take advantage of the latest developments in electronics, without the need for built-in obsolescence. This would require upgradeability through dedicated modular components, separating the photographic from the electronic capturing units. While such an approach would also work with traditional M's, it is likely that a newly-designed range, optimized for digital, would be required in the long-term. Looked at in the right context, Leica's expertise (and German engineering background) should be really able to develop such a unit in-house.
Selling the range would be relatively easy-the main values and qualities of Leica-made products would be maintained, however with an assurance of forward operability and acceptance of future technological updates. With an exciting and well-developed design, sales will far exceed anything possible with an M8/M9.