I am not sure I got the first issue, you can rotate each tile individually before placing them.

That's the issue. I don't want a sideways room tile, and for pure cube tiles rotation is only an annoyance.

based on any provided 2d shape would do that. We can find a way to make them fit in a grid with some roundings

Sure. I guess this would work but I dislike how custom of a solution that seems to be for what I'd say is general usage.

I just want bitwise calculation 1x1x1 to 8x8x8, so I can place down a chonky arena wall (8x8x2) in the same grid as a hallway (1x2x1) etc without risk of double-occupying cells. The only time I'd want that is for non-block models that are designed to occupy the same space (different types of wall decorations).

Autotiling would be nice, but that's a step up. Similarly, cell types like on_wall, on_floor, and on_ceiling (and in_air/not_on_floor?) could (if their cell type is empty) allow autorotated placement (similar to the raycast placement added not long ago).

These are the sorts of thing that would go in a proposal, possibly someone has already made these suggestions. If not, I am sure someone else would have better results writing it that myself. EDIT: proposal 1713