You hopes would be in bein - alll the bands ahve some sort of problem with each other. he Megadteh-Metallica thing still rageson since Lars and Mustaine are whiny sods, Slayer aren't exactly best mates with Mustaine, and everyone tends to pokefun at Anthrax's recent outputs sadly.
As for te aforementioned problem of Slayer overshadowing support acts, thats just how it is - if you have Slayer, Testament, Machine Head and Evile playing the same show, 99% of people there would only want to see Slayer. Regardless of how good the other ands are, Slayer fans will only want to see Slayer - I've experienced this first hand at the Unholy Alliance 2. Even people who were wearing In Flames and Bodom shirts didn't give a crap about those bands, they boo'ed nearly everyon until Slayer came on. They're a band ith such a massive (and slightly unserve) reputation that even if they were to play with Metallica, a majority of the people would go and see them.
Its really simple,you just have to poke around the metal scene a bit to really get it. This may seen like mindless bable to commonfolk but trust me - if you put two awesome bands on tour, then announce Slayer, people will only go for Slayer. Thats smply how it works ith them.