If you already have the 6x24 and a good 12.5" square, that's going to pretty much be all you NEED. There are very few things you won't be able to accomplish with those two rulers. Specialty rulers will make certain shapes *easier*, but I'd say you have the basic "needs" covered. So that's pretty cool!
If you're a gadget person and want more: I recommend checking out the June Tailor slot rulers if you're going to be sewing a lot of smaller squares, rectangles & strips. If you're going to sew a lot of triangles, a 60-degree triangle ruler will save you some time. If you do a lot of flying geese, a flying geese ruler is probably going to help out. If you're going to make a double wedding ring quilt, you probably want to check out some of the ruler sets made specifically for that pattern.
They make specialty rulers for just about everything - the question becomes how much do you want to spend, and how many rulers do you want to store that you might only use rarely?
Personally...I love rulers. I have several I have purchased because "some day" I'm going to make a quilt that has [whatever special shape] in it. I've used most of them, but not all of them. (yet!)
My 6-1/2" x 24" ruler BY FAR gets the most use, though. I use it on every single quilt I make, if for nothing else than for as a guide when I'm squaring up. Sometimes that's the ONLY ruler I use for a quilt!