Need to reharmonize the viola, but how would I know the ideal harmony?

So, in a composition competition on Musescore, I have to compose a piece for only strings. Now, the host did say that piano and harp were allowed but I was like:

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You know what? I'll limit myself to the bowed strings anyway. It would be way too easy to write a piano duet and call it done, even though technically speaking that would abide by the rules as the piano does involve strings in it's mechanism. Same goes for harp and for many of the same reasons as piano.

But knowing all the difficulties I've had with string quartets in the past(mainly getting past the first 1 or 2 themes is what's difficult for me in string quartet writing), I decided to do a trio as that I find much easier. It's like a 2-sided exponential curve, except not really, this is what I experience in string only ensembles(only exception is the last one):

So, as you can see, for me, Trio is one of the easiest and Quartet is the hardest. Thus me, who has had major difficulties with quartet goes for the Trio. I think I know why Trio and Octet are very similar in difficulty for me. Equal distribution of the roles amongst instruments. In others, it's more skewed as either 1 instrument has multiple roles(Duet and Solo) or multiple instruments have the same role(Quartet). As the ensemble becomes larger though, this spike become less and less prominent, thus Quartet being harder than String Orchestra on my graph.

Now on to the topic of the post. The piece I'm composing is a nocturne. I named the piece Nocturne dei Fiori(Italian for Nocturne of the Flowers) because I find that Eb major has this very beautiful, flowery tone, especially in a nocturne. I know, I don't have any dynamics yet. I will add those later(outside of piano scores, dynamics tend to be one of the last stages for me). In the first 6 bars, I have the cello take on the bass role, and I have the violin come in at bar 3 as that allows for an iteration of the naked bassline before it becomes harmonically embellished. In bars 7-12 the cello starts to take a melodic role rather than just a bass role as before. I think that if you isolate it to just the violin and cello, I've got some pretty good counterpoint going there(very few times when note values are shared(there's only 1 measure in which they are exactly the same in note values, and even then, the eighth notes are in opposite directions), contrary motion, etc.), but while the viola isn't bad for the cello by itself, I hear quite a crunch when they are combined with the violin, which suggests that I need to reharmonize the viola, as it has been the primary harmony instrument in the nocturne(what with all the arpeggio figurations that are so typical of a nocturne). But the thing is, how would I know what harmony is ideal? Harmonic change so far has been on the half note, might change it to on the quarter note after this to add just a bit more tension that leads to a minor key section.

I mean, what if the on beat notes clash a bit(C and D for example) but not so bad because of octave difference? What if the harmony suggested by the cello is completely different form that of the violin? Both of these actually happen in bar 8, the cello suggests Bb major on beat 1 while the violin suggests C minor on beat 1. How would I tackle that beat in that bar? Have an F minor chord instead of either Bb major or C minor? Here's the piece so far and I would appreciate the advice as to how to go about reharmonizing the viola to go well with both the cello and the violin.

Me personally, I don't like cello bass lines and viola arpeggios, I think it only works well on keyboards, and you have a lot of cross-voicing between the viola and cello, I would avoid that. In terms of re-harmonizing:

- bar 2 sounds weird because on beat 3 there is a V64 that should resolve to I6, but it doesn't, so it feels wrong to me.

I see that you are trying to make a I V VI progression, but the better way of doing that is I V VI6 or I V VI(if you can avoid the parallel 5th.) 

- I think that bar 4 was a missed opportunity to use the circle of 5ths with the bass going down in 3rds and up in 2nds

-bar 10 beat 4, the F in the cello should resolve to Eb on beat 1 in the next bar(my opinion)

I don't think implying different key centers is a good idea unless you modulate into the different key, to me, I hear 2 keys at once, which is kinda putting me off. When I hear a melody that is implying a minor key, I want the bass line and harmony, to suggest it too.

I think that if you got the harmony and the bass line nailed down, it wouldn't sound awkward.

I don't know if this is what you wanted but I hope you don't take it the wrong way, its just my opinion. 

That graph is really weird, how can a piece for solo instrument be harder than a symphony?

19 minutes ago, Ivan1791 said:

That graph is really weird, how can a piece for solo instrument be harder than a symphony?

 

If you leave the strings to a second place, or background most time, it's much easier to manage than if you have to give an original and genuine voice to a solo instrument (or a quartet).

If you don't master counterpoint, it's easier to write for three instrument than for four.

On the other hand I don'r understand well the focus here. The intention is to build a chord progression that goes well with the viola or the opposite?

In this style, avoid sudden and occasional clash by seconds (in strong spots), it would sound as a mistake.

8 hours ago, Luis Hernández said:

If you leave the strings to a second place, or background most time, it's much easier to manage than if you have to give an original and genuine voice to a solo instrument (or a quartet).

If you don't master counterpoint, it's easier to write for three instrument than for four.

On the other hand I don'r understand well the focus here. The intention is to build a chord progression that goes well with the viola or the opposite?

In this style, avoid sudden and occasional clash by seconds (in strong spots), it would sound as a mistake.

 

The intention is to make the viola harmonize well with the cello and violin, that's what I was talking about when asking about what the ideal harmony is.

And, there's no way that I can completely avoid voice crossing with those arpeggios without changing the register of both the violin and the cello, plus, voice crossing is to be expected in String trios and quartets, so I don't see a problem here with that. There would be a problem if I changed the register to avoid voice crossing, actually 2.

1. Cello would go from it's soulful range where it is now to it's Double bass like range, and I would lose that soulful quality and it just wouldn't sound like as good of a nocturne.
2. Violin would regularly get up into the third octave and I really want to limit the amount of third octave notes in this nocturne because the third octave can be screechy if the violinist isn't careful and a screech would just ruin the nocturne.

All the instruments are in their ideal registers here and that's better than changing registers to avoid voice crossing and thus ruining the nocturne.