I ended up finding a batch of old 25mm fantasy miniatures, which opened up a Pandora's box for me.
Taking a look at some of the older miniatures, I noticed that except for the size of their heads, they were actually pretty close to 1/72 in proportions. These figures would be a good starting point for conversion to 1/72 fantasy figures. I stripped the paint from these figures with Simple Green to save myself some embarassment. Starting from the right of George: Elf Princess (Ral Partha 01-021, first version), and Shield Maiden (Ral Partha 01-023, second version), both sculpted by Tom Meier; Fox Spirit, from the Oriental Werecreatures set (Ral Partha 01-119); Medusa, from the Female Creatures set (Ral Partha 01-087); Medusa, from the Fantasy Lords Monsters of Mythology boxed set (Grenadier 6004); and Harpy, also from the Female Creatures set (Ral Partha 01-087).
In order to do the conversions, I needed to find some properly sized heads. Here is a line-up of various newly purchased figures, starting from the left: 28mm Female Demon (Reaper 03084) sculpted by Werner Klocke; 28mm old style Satyxis (Privateer Press 29296); 25mm Elf Princess; George; 20mm Damsel in Distress (Elhiem M08, two other poses can be seen at the Elheim website) sculpted by Tidyfigure Maker; 18mm Elf Sorceress (Eureka 300HEL07); 15mm Amazon (from the Armies of Arcana line, formerly of Thane's Games and now available from Napthyme's Role-Playing Den/Lone Gunman Games) sculpted by Sandra Garrity. The 18mm and some 15mm heads would be perfect for 1/72.
In trying to find a source for heads, I also found that the bodies of some 28mm gnomes and halflings could work for 1/72. To the right of George: D&D CMG Halfling Slinger (War Drums #18); Halfling Thief (Reaper 02676), Gnome Bard (Reaper 03196), Gnome Sorceress (Reaper 2721), and Female Gnome (Reaper 2772), all sculpted by Sandra Garrity; Gnome Wizard (Reaper 03340) sculpted by Werner Klocke; Elf Princess. The CMG miniature is probably supposed to be 28mm scale, but is fully a head smaller than the metal halfling.
In the case of creatures, like these unicorns, they are suitable for 1/72 as is. The rearing unicorn (Superior WL-019) would be a good representation of the modern concept of the unicorn (more horse-like). The smaller unicorn is from the Reaper foals pack (Reaper 02207), and would probably go well with 15mm miniatures, but will be converted into the type of unicorn described in The Last Unicorn by Peter Beagle.
And speaking of 15mm fantasy, I never realized this before, but there is actually quite a large selection of fantasy figures in 15mm. Many of them could be easily used for 1/72. Here is a Reaper bear, and some Bear-folk from Splintered Light Miniatures. Most of the Splintered Light line seems to be sculpted by Bob Olley.
Some Bugbears, also from Splintered Light Miniatures. Other than the OOP Ral Partha 15mm Bugbears, I don't think there are any other Bugbears in this scale.
Some Hobgoblins from the Ral Partha AD&D Battlesystem line. The only 15mm Hobgoblins as far as I can tell.
Some Kobolds from Splintered Light Miniatures (larger kobolds) and Ral Partha AD&D Battlesystem (smaller kobolds). Again, other than SLM and OOP Ral Partha, there are no other Kobolds in this scale (as far as I know).
Needless to say, my workspace has not been cleaned.