Are there any plans for future referring 3d liquids? At current point it is usable only when monsters can not enter it, since they look rather... mmm... strange walking on "river's" floor. And water-dwellers are most unusable in this situation.

AFAIK, 3d liquids are implemented now (3d floors), unless you mean something different, as for avoiding monsters to walk in liquids, you could flag the lines of the water sector with monster block.

Well, I just thought it would be great if some water-dwellers, like Stalkers in Hexen could swim in the water... but this surely makes it necessary to have additional sprite frames if we are using 2d animations, or model anims if we are using 3d models.

Ah OK, so you are suggesting something like a flag for "flying" monsters to not cross the ceiling of the 3d water sector? or something like if the monster "crosses" the ceiling of this sector it will become the original stalker? The first one I remember the Edge port already used something like this flag, as for the second, it might be difficult to implement, but I'm not sure.