i agree with that.there is not enough variety. however there are reasons as there to ensure full usage of the data itself. this does limit the options that can be offered.
Three network do an All you can eat data sim only deal with 200 minutes of texts for £23 per month - this also allows 30GB per month for tethering. At present, there is also a free amazon voucher and quidco too, this may take the price down to around £17 per month (of quidco tracks and you would use the amazon voucher too)
I think you missed out a zero. £150 might be a more realistic price point for truly unlimited data. As most people who max out the data on the £20 goodybag probably use almost exclusively data, I would expect to get one with the same "always on package" for the same £20 price.
In any case, this consittutes an Idea, and one that has been repeated many times, so you should simply find the original occurrence and add your vote for it.
Unfortunately, giffgaff can't offer unlimited data. They do not have access to unlimited bandwidth on the O2 network, so it is simply not feasible.
They could potentially offer more data, but it wouldn't be unlimited eg. 12GB for £30.