For example, here's a fission-fusion one that looks like it only needs engineering to be workable:

https://www.nasa.gov/general/pulsed-fission-fusion-puff-prop...

In fact I meant it in a general sense, and both ion drive and a more classic confinement engine like design that expels the fusion products. I don't know enough about the intricacies to argue for one vs the other, but fusion fuel is the most energy dense fuel we have within reach, by a huge margin. The variable then becomes the mass of the engine (and supporting plumbing), but we have control over this.

Can we shrink a fusion reactor small enough to make the rocket as a whole feasible? I don't know. My hope is that for space travel, we can make certain tradeoffs that are not feasible on Earth - e.g. expelling radioactive waste products into the environment. A rocket engine doesn't need to be all that efficient at generating electricity either, it mostly needs to be efficient for propulsion, which might make it's design much simpler.