Until the Particle Emitter Nodes is implemented, this is a limited method using stock nodes.
It contains multiple compositions, for fixed speed particles or random speed, wall (top-bottom), center-to-sides (XY or XYZ).
Did not have the time to name them properly, but thought I should upload it in cases it helps anybody.
Any improvements or easier methods welcome !
PS : For some particle effects on start a first burst of particles is visible, I need to try to fix that, haven't had time to yet though.