I don't remember Gloin being in LOTR, just a reference to him when Gandalf refers to Gimli as "Gimli, Son of Gloin", nor do I recall Lobelia either (unless they were both in the book version and and left out of the movie just like Tom Bombadil).
But fair enough, if you wanna stick to the book, then they should at least get Hugo Weaving, Ian McKellan, Ian Holm (if age isn't a factor), and Andy Serkis to reprise their roles of Elrond, Gandalf, Bilbo, and Gollum respectedly. Also get John-Rhys Davies to play Gimli's father Gloin, although I doubt he'll do it, cuz if I recall, Rhys-Davies was allergic to the dwarf make-up, so I dunno if he'll wanna go through that hell all over again.