Yah nice in theory, but that could make sorting out groups harder. For example, our guild almost never had mages in it; we are 5-8 actives, at max, most were rangers, up to the point we went with all-ranger groups at times and in our timezone is no alternative to that, the server is not very full in general. No people got mage as an off-class either. So we could not even play content which is immune to the skills of a whole class; or not on the highest levels and at some point only these are interesting enough for people to play at all.
Also players would hit a rock who only got one class (yet); there’s classes way advantaged over others already, due to boss mechanics alone; it is bad enough that shamans are pretty much useless, for example, at Sewer Slime boss and others, so people only playing that class are already having a hard time in groups.
The other thing not compatible is the whole tileset system, most endgame players work out a special, nearly perfect rotation and you have to use it, just like this, to make the tiles proc, can’t just switch it out to this and that… so yea it would make the game very different.
Not really sure what would be different with that? Classes already got own strengths and weaknesses which show in nearly every fight… up to the point that I am switching to classes which are more effective, be it for mobs in the overworld (some mobs are a pain with shaman, for example some flyers, some melees are way better), but also in dungeons.