The 737 coversion, the Be-200 and the Il-76 'projects' have all been discussed (although I'm not sure that's the right word - you think PPRuNers are rude
) on the Tanker Pilots'
board. Unfortunately they don't archive posts, but from memory the 737 had insurmountable structural limitations - centre wing box IIRC. As to the Russians, it'll never happen IMO. Neither type is currently certified in the US (but most of the US tankers aren't either), but that's a small impediment compared to overcoming the NIH syndrome of the US authorities, various fire agencies and most of all the above-mentioned pilots. Most of them think jets are the devil's work, air tankers must have round engines - preferrably incendiary Wrights
The sheer testosterone factor of the prospect of a 747 dropping mud from 100ft might be convincing enough to get one done. Long-term solution ? Doubtful.