by Clipper
The expansions are what you want for more enemy variety. Some of the Villain packs will give you non-unique figures that could be used in the campaign, but those figures are generally included in the expansions as well (albeit as cardboard tokens rather than plastic).The bigger box expansions generally have more figures in them compared to small box ones, but each expansion will give you some extra variety regardless of size.
Some of the figures from the earlier expansions used "Habitat", making them unusable in some campaigns (for example, you'll only get Wampas selectable during missions that have snow tiles, meaning Hoth missions). Those are a tad rare, and you could always just ignore the "Habitat" ability as it isn't really used any more on new figures (the devs realised that restricting figure usage, while thematic, is less fun).