Our Bridge of Don bathroom extends into a former cupboard and repositions the vanity around an L-shaped bath. The bath has a broader showering end, while the clear screen leaves the tiled surface in view. The useful lesson is that gaining floor area opens several decisions at once. The new boundary can affect where the bath ends, where you approach it and where the basin fits.
If you are considering taking in a neighbouring cupboard, draw the existing and proposed boundaries on the same plan. Then mark the clear space needed to approach the bath and stand at the vanity. Check which of those tasks becomes easier and where the old cupboard's contents would go. That gives you a way to judge the benefit of the alteration against the storage you give up. It can also show whether relocating a fitting is doing as much work as the extra area.
