The practical limit in terms of sky coverage is probably given by funding. The HDF, according to wikipedia, covers a 24-millionth of the entire sky. We would have to launch thousands of Hubble-like instruments to cover the entire sky at that resolution and sensitivity. Much of that area is covered by the Milky Way and and gas clouds, anyway, so not all deep exposures will reveal as much information as the HDF. Will we do this? Probably not, however, there is a lot to be learned from very precise whole sky maps because galactic shapes are being distorted by the irregular gravitational fields in the universe. This leads is to weak gravitational lensing. Proposals like the Euclid mission will use specially designed space telescopes with visible and near infrared capabilities that are similar (not quite as high resolution) as that of Hubble to map areas of sky that are hundreds of times larger than those that Hubble's very small angle cameras can take.
If we want to go earlier in time, then we have to use near- and mid-IR and eventually deep-IR telescopes. The first of these will be the new James Webb Space Telescope, which is the mission that will augment Hubble with the necessary IR capability. If you have been following the JWT in the news, you may have heard that in terms of funding it is very close to breaking the bank... it's a very ambitious and expensive instrument that eats into the budget of a lot of other potentially equally interesting science missions. For the foreseeable future it's the largest space telescope investment that we will be making, which brings us back to the funding limits...
