Like 'Wot's, Uh... the Deal' (about growing old) and 'Free Four' (about the same thing) which would both fit on Dark Side pretty well. One could also stretch the theme of 'The Gold it's in the...' and fit in into 'On the Run', about not being able to settle down, for various reasons.

And I always saw 'Mudmen' is a kind of counterpart to 'Great Gig', except without the vocals (which were added in the studio anyway). Both are ethereal instrumentals with build-up parts.