There are two potential issues with pants:
1) Male Garb.
2) The “separation of the legs” is visible.
The halacha regarding (1) is that male garb is determined by society. Therefore, in a society where women wear pants, pants would not be considered male clothing.
(2) was an issue raised by the Chazon Ish, and that would apply even if pants are not an exclusively male garment. However, it is only an issue if one interprets a certain Gemara extremely broadly, and that is debatable. I know rabbanim who do not agree with this interpretation, and thus do not agree that this is an issue.
Some say that pants are not tznius because they show the form of the legs. I do not think this is a point about pants. Obviously, any article of clothing a person chooses to wear should be chosen with discretion, whether it’s a pair of pants, a skirt, or a hat.