Use recycled brown paper bags. The Lego is fine being plastic as Lego doesn't get thrown out, mine had been passed down to my cousins and brothers. This would also look good for Lego as they would be seen as more eco friendly. 

As Lego is very popular and is widely available in many countries the bags in the sets that are being thrown out stack up. If you really think about it that's alot of waste that could be prevented. 

At the moment Lego will not do anything to be more environmentally friendly, but if we ask them to do a small change like this they might consider it as they would be making so many people happy, possibly bringing in more sales.

It is a no brainer for their marketing team to pull a stunt like this, as many journalists will take note, creating articles that will spread the word. Maybe other big companies will follow suit.

A change like this wouldn't be hard to pull off as I'm sure it's easy to get in contact with recycling companies who will supply recycled mulch in bulk, which can be bought and turned into cheap paper bags. Like oatmeal packets.

I think you should support this cause because we are all going to die soon due to climate change, so let's go out while trying to do things about it. :)