I think the 35 Sonnar FE is the only option. It has the added bonus of weighing only 120g.
I was not planning on switching into the A7 system, but I am increasingly of the opinion that thinning out Leica kit to pay for a switch (and having money left over) is the best bet. I'm priced out of Leica's future anyway, so why retain lenses for a system whose manufacturer cannot make a body that uses the lenses for under $7K? At least I have a future with Sony and, even though the M240 or MM may equal or even outperform the A7R in some areas, even if I only buy into alternate sony generations, I will be able to buy into Sony development for the next decade for the price of one Leica DRF body now. Switching systems will free up cash, even after the switch.
Absolute optical perfection is not necessary for most people most of the time. Heck, it was not even present in all of the Saint Ansels images, some of which were not enlarged heavily due to corner softness, or wind vibration. I was not surprised to see the Leica 50 APO Summicron, but did feel it confirmed that Leica was heading in the wrong direction (still) by concentrating on exorbitantly priced perfection rather than getting cameras and lenses into the hands of talented photographers. In the past, I think they were more practical and less elitist, but the financial success of their boutique approach cannot be argued with. The company has to survive.
Here enters the A7/R. It lacks in some areas, but is dynamite everywhere else, and for a fraction of the cost of the M240. Its the 90% solution and the one I have been waiting for, personally. I could not care less if the 35 FE Sonnar is not quite as sharp on the edge of the frame as a 35mm Cron asph/Summarit/FLE on a M240 under close inspection, because its gonna be close enough not to really matter. The total buy in for that pair is (in the UK) £2329 (Sony A7R and 35 FE) and £6300 (Leica M and Summarit). Nuff said.
Sony is going to rake in the cash from these new Alphas and will reinforce success with more R&D and great new releases. This is just the beginning. If rumour is correct and Zeiss produce an equivalent of the ZM line for the FE system, we can expect astounding edge performance, nice manual focus and far lower cost than leica glass. Even selling old Leica lenses will probably more than pay for buying in new. The only areas I see the alphas being a little weak is in street photography, with the quick release of the Leicas and the OVF.
Lots of waffle, but thought Id share where my thoughts have been going.