My thoughts were along the same line as flars - 5 gallon batches, which requires a fair investment in equipment, but is very little more effort per batch than doing one gallon. But seeing as that doesn't hold appeal for you at this time, then there are two possible suggestions I would make:
1) A 2 gallon bucket. This will allow you to have more than one batch in process at a time, which allows you to experiment with more beers. And it is a very cheap upgrade. Alternatively, you can instead get a second 1 gallon jug with airlock or blowoff, but I'm a big fan of plastic for this purpose.
2) Look into fermentation temperature control. This will do more for the quality of your beer than any other upgrade you do. There are many different ways to handle it, and some of them can be pretty expensive. But the cheapest option is called a "swamp cooler", which in the case of beer is simply a shallow pan of water into which the fermentor is placed. You then drape a t-shirt over the fermentor so that it wicks up the water, and the evaporation off the t-shirt causes the fermentor to cool.