External map maker.

Score + 1
Maybe having an external map maker that people can use to generate a background map and save it as an image on roll20 or on their computer to upload could allow people to make complicated maps without having to worry about the lag in-game. 
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Their are a number of these editors already available. I personally use Dungeon Painter Studio by pyromancers, but they do have a free version on their  website . I recommend the Studio version available through Steam because it allows you to upload art assets, such as art packs you have purchased on roll20, use them in your maps.