[xiphias]

Dynamic block size makes it harder to reason about security of the system, so it's not a conservative choice. Adding another curve for signing won't be hard with segwit, the only question is if there is really enough demand for it (and it doesn't need a ,,switch'', both curves can be supported at the same time, and users can select which they want to use). One of the most interesting ideas right now that may be implemented in the future is mimblewimble, unless something better comes. But there are many other, easier to implement ideas talked about on the scaling bitcoin conferences.