In science-fiction, engines that can accelerate at a constant 1g (more or less) are called torchships. Interestingly enough, there are theoretical designs that achieve that and give us huge amounts of ΔV and still large amounts of thrust.

An example is the "Plasma Jet Magneto Inertial" [1]

The main issue that may not be solved by eventual iterative progress is that these kinds of engines create way too much waste heat.

[1]: http://www.projectrho.com/public_html/rocket/enginelist2.php...