I find it's good to not prep plots, prep situations (I think Justin Alexander has a blog post called that) - and in fact I generally try to make the starting situations simple and easily comprehensible to the players, because they can only act on information they are aware of. A convoluted situation the players cannot understand is worthless to me, a random encounter generator would take less work and produce more fun results.
Starting with simple initial situation, I put events in motion as the PCs enter the milieu. With NPCs having conflicting goals, complications will rapidly emerge. Factions will clash; the players will have opportunities to get involved, or to choose not to get involved. Some conflicts may resolve off-screen; others may directly impinge on the PCs wherever they are.
In my recent campaigns I'm trying hard to avoid "this is the plot/you must do this" linearity, a good tactic is to always keep open the possibility the PCs walk away from the current situation (even if this is unlikely) and consider what will happen if they do, and what is going on elsewhere they may run into.
So, where added complications/complexity do arise, I like it to be organically, eg from the interaction of these particular antagonists either with the PCs, or with other factions. I try to avoid top-down plotting of the "hm, must insert a complication here" kind, that tends to feel artifical and forced. Whereas "Hmm, I wonder what X is doing while this is going on with Y...?" type thinking leads to complications that arise naturally in play.