I don't know how difficult it is to implement, but since it has been possible in Deja Vu for many years, as well as in MemoQ, I think it would be good to finally make a TM usable in both language directions. This of course would apply to secondary TMs (big mamas) rather than the main, project TM.
I've always thought it was a limitation within Word-based CATs (wordfast e.g.), but standalone programs should cope with it and not rely on AnyTM solution, which basically involves creation of a new TM.