Recent developments in designs for e+e- colliders

We discuss an design of future e⁺e^- collider from the view point of the beam dynamics. The crossing angle permits short bunch spacing without complex design of the interaction region. In KEKB, the crossing angle did not degrade the collision performance. However we need to study how the crossing angle affects the beam-beam interaction for the future high luminosity colliders. The electron cloud instability, which limited the luminosity performance in positron ring, have been somehow recovered by using solenoid magnets. The ion instability, which have been observed in electron rings, may be serious for higher beam current. We have to study which particle should be stored in high or low energy rings to minimize the instability effects. The coherent synchrotron radiation becomes serious for short bunch length and high bunch current. We discuss these issues one by one.