My vote would be for WE open bolt (or closed bolt - it's really only the feed lips, gas seal bucking and follower that differ) GBBR refillable CO2 STANAG type magazines. Ideally, these magazines would have an integral CO2 reservoir instead of using disposable 12g cartridges. ProWin make such a thing for the WA system but there is no such thing as an 'after market' magazine for the WE system.
I would think that with the rising popularity of WE GBBRs that such a thing would be a decent seller ... but's that my humble opinion as a WE GBBR user who prefers CO2 over propain.
Failing that, perhaps a conversion kit that would enable one to use the aforementioned ProWin mags in a WE gun...