When you build a pyramid in DF you have the choice of making it an even number of tiles wide, or an odd number. This results in the top of the pyramid being 2x2 or 1x1 tiles wide respectively. Currently there is no way to create a capstone to have a properly pointed top to the pyramid, if you place ramps on top they will simply appear to be flat because they are invalid without an orthogonally adjacent wall. So my suggestion would be to add new graphics for ramps placed in 1x1 and 2x2 squares with no supporting wall tiles. A single ramp standing on its own should display a four sided pyramid. A 2x2 square of ramps should combine to create a larger pyramid using four corner ramp tiles.
This would not be difficult to implement because the graphics for this partially exist already. 1x1 ramp tiles are displayed as a pyramid, but only if they are carved out of natural soil instead of constructed. The graphics for 2x2 ramp pyramids are four of the existing corner ramp tiles put together, since this is purely a cosmetic change you would just have to tell the game to display them this way, there is no additional gameplay effect.
Another minor graphical issue is that the existing single-tile ramps have black dots at the corners of the tile, which are even more noticeable when several ramp tiles are placed beside each other. These dots could be recoloured to match the ramp and do not seem to serve a purpose.