'flirting' With Keys

Hey, I am currently composing for a string quintet and I have encountered a theory block and don't know how to go about it. I'll do my best to describe the passage and what I'm trying to do.

OK, so, my original phrase consists the bass line descending (I'm in Ab minor), so it goes Ab, Gb, Fb. Without thinking, I was playing this and then instead of carrying on down to Eb a went up to F natural. It sounds really nice and... 'eery'.

So, what I am trying to achieve now is extending the phrase like this: Orginal start of phrase (Ab min) - Sudden key change (Maybe for only a couple of bars in F min) - Back into the original ending of the phrase (in ab min but because it orginally occured halfway through the phrase it goes back into it on what I think is a Gmaj chord but with a 6th instead of a fifth (Eb, G & Cb)) Apologies for my terrible theory.

To try and help clarify more...

Original phrase bass line: Ab, Gb, Fb, Ab

Extended: Ab, Gb, Fb, Fnatural, ?Stuck here?, Eb, Ab.

Oh hell, that is terrible way of explaining but I hope you will get the idea and be able to help me.

Thanks

Db, D or Bb would be fine. And that's not a G major chord with a sixth instead of a fifth; it is an E flat major chord – the dominant – but with the fifth replaced by a sixth.

Bass - ab: 1 b7 b6 1 ~         f: ~             ab: 5    1 b7 b6 6 (7) 5 1
Harmony to the extent it's mentioned -        V+    I

Not sure if I'm understanding the question but G natural is what I'd put in between the question marks. If this is a bassline question rather than a flirting with keys question it forms a functional melodic minor scale but I guess I'm missing the point comepletely.