If you parse through drugsdata for example, you can find some of these. The adulterants were typically MDA, methamphetamine, amphetamine, caffeine, 2C-B, mephedrone, metaphodrone, MDEA, cocaine (often in orally active amounts), piperazines (typically in mixes), ketamine (though not in orally active amounts), and one sample that I found including DOM. Most commonly though were MDA, caffeine and methamphetamine.
Check this out for an example of some. The blue dolphins appeared to predominantly contain methamphemine alongside MDMA.
I've personally mixed MDA and MDMA with a ton of other substances, and find the best combinations to be miprocin, 2C-B, allylescaline, 25C-NBOMe, DOM, amphetamine/methamphetamine, and LSD, at least as far as things that I could orally fit into a tablet (25C-NBOMe is often said not to be orally active but I've consistently found that not to be true, as well as others around me). 2C-B, miprocin and LSD create very psychedelic and jovial experiences, allylescaline made the rolls very psychedelic but also peaceful and serene. DOM, amphetamine and methamphetamine turned MDMA/MDA into what I often refer to as "turbomolly", profound stimulation alongside a normal roll.
Orally active dissociatives may make interesting mixes here too, 8-12mg of 3-HO-PCP, 15mg of 2C-B, 75ug of LSD, 75mg of MDA and 50mg of MDMA would make a mixture that I bet would be absolutely fascinating.