There's been some wild speculation going on here....
Here's the drum as far as I can see it: there's technology sharing between many German mfr's. & has been for some time. Given the competitive nature of the current marketplace & competition from far eastern mnf'rs., not to mention changing customer expectations regarding price & longevity, it makes sense.
Obviously, Bosch & Mafell have contracted to share tech. in their tracksaw and 36v platforms, & Metabo has likewise with their 18 & 10.8v platforms. This helps reduce overall R&D expenditure by amortising cost in a larger production run split between different brands/companies, and makes more efficient use of their individual production capacities.
Some medium sized enterprises will no longer play ball, however. Festo in particular, now seems to jealously guard their products & acquisitions, being less willing to share. A case in point is their acquisition of Reich, makers of the old Holz-Her branded power tools. Reich once manufactured the majority of Germany's Belt sanders: Holz Her, Metabo, Atlas Copco, Milwaukee, AEG, Wurth and even (originally) Bosch 75 x 533 belt sanders all emanated from Reich's factory. It appears that since Festo's acquisition all contracts have been cancelled & the others have had to source alternative designs elsewhere.
Due to the contractual nature of these marketing & technology sharing arrangements, one could conclude that Mafell still have quite some time to go with their 36v Bosch contract. Maybe in the medium to longer term there may be some changes, but once locked in a contract is inviolable. Any rumours of a change in contract partner is speculative at best in my opinion.
This is rather unfortunate, as Metabo seem to be the current leaders in "German" (actually Korean) battery tech. Samsung make the actual cells that Metabo uses. Metabo have some excellent cordless tools available; just look at the current Mafell range of drills. While the range is smaller than their asian competitors, their quality (in my opinion) reigns supreme. The best cordless drills, impact hammers, grinders & jigsaw available, at least in the 18v class anyway. Plus the little tiddler drill & the lovely little mini saber saw are winners too.
It's not necessarily all bad, however. Bosch have also upped the ante in offering an all-new "semi" compatible 36v battery platform. These 36v "professional" batteries have 6.0ah capacity & an all new ultra-fast charger. They're big, powerful and (heavily alloy finned) heavy, but like Metabo's next-gen. batteries have heavy internal metallic bus-bars and new cell chemistry to allow high current flow in (for much more rapid charging) & out (for high current loads), which is where the extra fan cooling and alloy finning comes in. Currently Bosch markets this platform only in their new "professional" range of garden tools: mowers, strimmers & trimmers etc., but I've heard some suggestion that despite the additional physical size & weight the actual interface is unchanged and MAY fit some or all of the current range of tools & chargers, which could include Mafell of course.
I've heard whispers about a pending new Metabo 36v range too. Certainly the new 36v grinder seems to be a game changer. Contrary to what has been stated above, the new grinder appears (at least in the photographs & videos I've seen) to use an all new discreet 36v LiHD battery. Whether it's as some speculate a 6.2, 5.5, 5.2 or other cell capacity is currently unknown. My guess is that it will use current Samsung cells, making it either 6.2 and/or 5.5 ah capacity.
I doubt any of their coming 36v tools will be able to use 2 x 18v as an alternative to a discrete 36v battery. It wouldn't make sense from a marketing perspective, as none of the current Metabo 36v range (SDS, Hedge shears etc.) would therefore be backwards compatible to a new 2 x 18v battery interface, whereas new 36v LiHD batteries can interface with older tools. I also cite the fact that all current Metabo chargers; ASC 15, ASC 30 & ASC Ultra are compatible with 14.4, 18 & 36v platforms as evidence of a continuing 36v battery presence.
While it would also be nice for them to make a battery sized and shaped transformer to run these tools off mains as well as battery power, I can't see this happening either. It's been tried before (DeWalt???) without much success, as it doesn't make economic sense. Why make it easy for the customer to decide on only ONE tool when there's the opportunity for additional tool (corded & cordless) and battery sales? If that's your bent, then what about cannibalising an old defunct battery of the right interface geometry and jury-rig one up yourself? I'm sure THAT'S been done before, too.
Metabo have developed some truly excellent brushless motors for their latest range of cordless tools. I especially look forward to them using the 36v grinder motor in a new 36v saw, SDS hammer & maybe even a SCMS. These tools could revolutionise the construction industry in a way that 18v tools (as good as they are) can't. There's just no substitute for power in tools with high current demand.
Metabo's rail system unfortunately isn't compatible with existing Mafell/Bosch FSN guiderails. It instead uses a (compatible but slightly different) variant of the Festo/Makita/Metabo rail. Their current saw plates would require heavy modification for compatibility with the clearly superior FSN system. Any future Metabo saws will unfortunately be likely to use the same old rather crappy system as before.