Concerning .ISO, .CAB, wlx, wcx... On the one side, there are a lot of plugins, so internal TC-plugins support could make MC development easier in the future.
On the other side, implementing such support might become just a waste of time since there are already at least 2 filemanagers supporting wlx and wcx (Unreal Commander, Double Commander).
I think you may just release own plugin sdk and concentrate on other global ideas. Step by step (maybe some vote again?). From my point of view, there's no need in fourth Total Commander. IMHO it is impossible to create something truly perfect that can completely satisfy everyone.
Don't think that implementing existing wcx, wlx, wfx and wdx plugins is just a waste of time. The question is not only in creating own sdk, but in fact in convincing 'lazy' developers that they have to write something under it. What for they have to write something under specific file manager on a new sdk, if, in fact, their plugins already work fine in extensive amount of other file managers? Different developers use them without any problems and if user want to migrate to another file manager familiar tools would be used.
P.S. FreeCommander XE also have support of wcx and wlx plugins.