Alternatives to those places. This would be places that are sort of, kind of similar or at least within the same country or region. Places which are "under visited". Places that most people aren't aware of and that do not get large crowds
I fear that the problem is that there are no "alternatives" to the most famous WHS. You wouldn't suggest going to e.g Chioggia "instead" of Venice - though you might suggest that, if you have already seen Venice, then you might find Chioggia an interesting place to go to on a further visit or for that extra day. Similarly - interesting though Avebury is it would be better seen "as well as" Stonehenge rather than as an alternative!
I see "alternative" sites as fitting potentially into the following categories
a. "If you have seen and liked that then you might like this next time"
b. "If you can't get to that then try this"
Have you come across the book - "The Road Less Travelled - 1000 amazing places off the tourist trail
" (Pub - Eyewitness. ISBN 978-1-4053-4427-2). You might find it of interest for your project - even if only to differentiate it! As well as WHS sites it covers a lot of "Non WHS" events/activities etc etc but does suggest a fair number of places which I would put into the above 2 categories. I have picked out a few WHS ones below. In addition to the main comparison it also lists other "similar" sites for each - so for instance a number of worthwhile sites of Prehistoric Stones, Chinese Temples, Roman Arenas etc etc. Much of the information is readily available on this web site via "Categories" and "Connections"!!!!
a. Pyramids of Meroe v Pyramids of Giza
b. Avebury v Stonehenge
c. Agrigento v The Acropolis
d. Herculaneum v Pompeii
e. Tikal v Chitzen Itza
f. Ponte delle Torre (Spoleto) v Pont du Gard
g. Puning Temple v Temple of Heaven
h. Annapurna Sanctuary v Everest Base Camp trek
i. Fernando de Noronha v Galapagos
j. Pula Arena v Colosseum
The general conclusion must be that none of these is a true "Alternative" - either the more "famous" is genuinely "more significant" or you should try to see both!!