I'm not a vocalist and I don't want to hire one. Instead of the obvious solution of either learning vocals or getting someone to do vocals for me, I've decided to make things more difficult for myself by trying to find alternatives to vocals in music. The main problem I have is that with most instrumental music it often feels like there's something missing, or that something needs to fill the space.

I've tried a couple of ideas - interesting synth sounds, guitar melodies, emphasis on changing rhythms, even sometimes resorting to using rhythmic harsh noise on some tracks that just needed to "go harder". But, I'm not sure about how effective this is when it comes to "replacing" vocals. Even when the song structures are more suited to instrumentals than to vocal-driven structures, it still feels like there's something missing.

Currently I'm making a blend different extreme metal styles with a little exploration in to electronic stuff here and there, so I'm not worried if something sounds off-genre.

Does anyone have any ideas? I know the "real solution" is to just do some damn vocals, but is there anything else that might be worth exploring? Even if it's just for fun?

none of it will replace vocals and still will sound like you just tried to fill the space instead of vocals.

Use brass instruments, they're the closest in capability of modulation you'll get to the human voice.

I used to play euphonium in my school days, maybe that'll do the trick

If you're not a singer you can make heavy use of effects, like distortions, chorus, vocoder, to alter your voice in creative ways.

Yeah, jumping off that point, a band like Spektr has pretty minimal use of vocals and supplants that with a lot of samples.

I think your best bet for an instrumental band is just memorable melodies. Especially lead guitar lines.

I doubt it's the style you're going for, but Opeth's "Ending Credits" has a lead guitar line that's just as memorable and effective as a vocal part. Maybe try something like that?

Have you considered spoken word? Black Flag tinkered with the idea one their album "Family Man".

This might sound far-fetched, but a saxophone is worth exploring for the fun of it. Also similar instruments like the trumpet, trombone, etc. I don't know how it would turn out, but I think these would solve the problem of whatever you're making sounding "empty". With an instrument like these you could manipulate the air flow of the recording, and it would bring a "human" (for lack of a better word") quality to the music. Another idea is to use a theremin.

The Chasm uses heavy lead guitar in place of vocals on their instrumental stuff and it works great.

I’m doing some spoken word parts, although not as a substitute for all vox. More to provide a change of pace / style to supplement the vox. But you could easily use that to convey thoughts at certain points in a song.