If a dimension is configured to reside above the End and therefore generates portals at the End's ceiling, then the Enderdragon and it's Bedrock fountain spawn above/at build height.

This problem might potentially affect other surface oriented post world generation features as well.
Also things like Galacticraft's space rocket may have issues when launched into a portal layer.

In addition to that, ceiling portals in void dimensions could generally cause additional light computation lag and memory usage due to breaking minecraft's "partially filled chunk"-optimization.

Solution idea:
In worlds that are open at the top, somehow only generate the ceiling portal layer when and where needed.