Regards to mesas, I don't think regenerating old mesas would really be a good idea. You get extra gold in them sure, but there are new mesas for that. New mesas lack one important feature of the old ones - really thick clay mountains. Old ones the hills are solid clay down to slightly below sea level, with big fat veins of undyed clay. The new ones the clay is only about 10-15 blocks thick on the surface, making them much worse for clay quarrying. Having both kinds of mesas is good I think as each serves a fundamentally different purpose. Old ones for clay, new ones for gold.
Mansions though... Yeah as soon as the border expands to contain one or one otherwise appears in the map it will be immediatly raided and all the illagers taken or killed. Without some sort of regen (anything) it might as well have not appeared because all it'd be is an empty structure. Might as well build our own one in that case. Not having regen here also puts players into direct conflict with each other - some (Like APB) want to get advancements, and so want to kill the Illagers. Others (like me) want to put them to work in various mad contraptions like a vex xp farm. If I managed to make such a farm I can imagine very clearly the hungry eyes staring at the Evoker powering it...
So no, border expansion would not solve the woodland mansion issue.