The best use of miles are F RT tickets on the fully integrated (LH, LX...) airlines because of the companion award.
RTW are (as has been said) not a good value.
You can also do One-Way tickets on fully integrated airlines.

South Africa is obviously a good suggestion.
Another option is South America, open Jaw for example to BUE and back from GIG, especially thru the northern hemisphere winter.

If you plan a longer trip and want to go RTW you could get Europe-Adia, then buy a one way ticket Asia-US and continue with a one way from the US to go back home. Good places in Asia to buy fairy inexpensive one ways are BKK, TPE and especially CMB. You most likely would also need to buy some tickets in Asia and US to be able to visit multiple places.