Judging from some sources, in particular
this video, it seems the expansion will actually make the game
harder in some ways.
In short,
each new big feature for the soldiers comes with a catch : rushing to collect the new resource, the Meld, put the soldiers at risk (or encourage the player to sacrifice non-valuable rookies...), and the mech-soldiers have a weakness, they can't take cover and are very expensive (death of the soldier also means destruction of the expensive mech).
There is also the new enemies, in particular the Seeker who comes with its own AI, that will use cloaking and targets your weakest or more isolated soldiers.
And at last, a few balancing changes will impact the difficulty of the whole game: some of the encountered aliens will immediately activate overwatch (or even try to take a shot with a Second Wave option), some skills have been nerfed (Squadsight will be unable to produce critical shots for example), soldiers promotions will be slower in all difficulties and same for research in classic and impossible modes.
From all the info we have so far, it seems the point of the expansion is not to make the game easier but to give the player more options, through both adding some new stuff (powerful but tricky) and re-balancing some old stuff (either because it was too powerful, like soldier progression, or because some choices were too one-sided, like going with only the Squadsight skill for snipers).