I have written full canons before and I have attempted writing fugues. But every attempt at writing fugues, at least 4 voice fugues leads to parallel octaves because I'm like:
Okay, I have reached an octave, now what? Going down to a 7th won't work because that will lead me back to the same octave. Going to a 9th also won't work unless it is between the tenor and the alto because I can't play a 10th interval with a single hand so if the 9th is between the tenor and the bass or between the alto and the soprano, my only choices are either to go back down to an octave or to leap. Going back to an octave seems like the better option because I conserve leaps, but it just delays the resolution of the octave. Contrary motion to a 6th might work, but what if that doesn't fit with the harmony? Only other solution I can think of that doesn't break the parallel octaves rule is a leap in 1 or both voices involved in the octave. And in general I want to use as few leaps as possible outside of maybe a subject entry, and if I have to use a leap, I would prefer a third over a sixth.
And I end up not finishing the fugue
Now I've been told that the distribution of 4 voices in a keyboard fugue is generally that the bass voice is alone in the left hand and that the other voices are generally in the right hand. But, it seems to me, that having 2 voices per hand just feels more balanced. I have also been told by a few people that fugues are easier to write than canons. Now this seems very counterintuitive to me. In a canon, you have the same melody, just delayed by a certain amount in each line. Sometimes this has a ground bass below it. I won't give an example from Pachelbel because you all know it. Instead, here is a 4 voice canon by Mozart played by a string quartet:
As you can see, every voice is playing the same exact melody until the cadential section is reached.
In contrast, fugues seem in all ways more complicated than canons. If I had to describe it as a cross between 2 forms, I would say that it is a cross between the canon and Sonata form. It has the main melody in multiple voices, just like a canon. And sometimes there are 1 or more countermelodies that also appear in multiple voices. There is also motivic development which is similar to what you find in Sonata form. 1 thing that differs the fugue from both canons and Sonata form is that the voices are like completely independent even if you don't take time into consideration. Time is what makes the lines of a canon independent. Fugues have lines that are independent through melodic shape alone. Sonatas rely on the relationship between melody and bass. No such relationship in fugues. In that sense, sonatas are closer to a ground bass canon.
And like I said before, I have never written a complete fugue, just a few complete canons. Even something like Fugue in C minor WTC I seems easy for a fugue but hard for counterpoint in general and especially compared to canons. Here is the fugue I'm mentioning and this video uses color to show the 3 independent voices:
So are fugues really easier to write than canons if they are almost like a cross between the canon and the sonata? If canons are hard, then wouldn't fugues be even harder?