Here's a great starter blend for a sweeter, cleaner espresso. The absence of North African or Yemeni coffee takes out a little bite from the cup and possibly some lurking fruity ferment flavor. this is, as noted above, a sweet blend used at a street level roasterie/caffe in Rome. They use a Guatemala Antigua for the Central:

50% Brazil Dry-process
25% Colombian Wet-process
25% Guatemala or other brighter Central American
I don't think Colombians really pull their weight in a blend (though many people use them as a base or part of their blends), and like using some Sumatra better:

50% Brazil Cerrado Dry-process
25% Guatemala or other bright Central American
25% Sumatra -Premium like Triple-Pick, Lintong ...,
Some sharp sweetness (Central American) hides behind the nutty Brazil flavors and the wonderful Yemeni aromatics. Mandheling adds body and depth. Yemeni coffees are fun for espresso blends, where they can be used like spice to give zest the aromatically or enzymatically flat blends. Roast to Agtron 40 to 35. Good crema production from this blend due to the many dry-processed coffees

40% Brazil Cerrado Dry-process
20% Panama or other bright Central American
20% Yemen
20% Sumatra Mandheling
Ah, too sweet, too boring. You want something more aggressive, chocolatey? Drop the Centrals:

50% Brazil Cerrado Dry-process
25% Ethiopian Sidamo or Yemen
25% Sumatra Mandheling Dry-Process
You can certainly keep going along this route by adding other coffees (monsooned, aged, robusta) to discover what they add and what they subtract from the blend.

For an potent Indian Monsooned-type blend you could do something like this:

60% Indian Monsooned Malabar -this high percentage will cup very musty
20% High Quality Robusta: Wet-processed Indonesian or Indian
20% Wet-processed Arabica, for aroma and balance: perhaps Indian, Timor, Java or Sulawesi.
For an potent Aged coffee blend you could do something like this:

40% Aged Sumatra
30% Sumatra, or Sulawesi
30% Guatemala or other bright Central American for aroma and balance
(Aged Java is very potent and should probably not exceed 1/3 of the blend or so...)