You can always combine certain parts of AddOn compilations (you can even take out certain AddOn settings by copying the lua file in the WTF folder).
The most important parts of the UI are the unit frames and the actionbars. Pick those two by choosing a compilation that has either or both of these with settings that you like and/or suit your playing style. Off course you can pick your actionbar layout very easily with the major actionbar AddOns.
Next step is finding out what parts of the default UI you find attractive or not, and supplement the above two with (an) AddOn(s) for that/those specific feature(s).
Then configure the added AddOns with a unified idea, same font, same statusbar texture, same width (for example a DPS/threat meter under the minimap), backgrounds/borders.
If the AddOn does not have enough configuration options/features, see if there is another one that does, or submit a feature request for it. A lot of authors can either accept the request, or give advice on what they recommend for it. Compilation authors however, mostly do not, unless they have included AddOns by their own hand(s). I am talking
about various persons here.
That's just the general idea in the wow UI community really
