I don't think Neville will have any bigger part in the future Potter series. He is just, you know, one of those stand by characters like those in the Japanese cartoons, who are only there to make blunders, and whose main characters come and save them and makes their own strength felt.
Anyway, Voldemort is not the only guy that Harry's gonna kill, I can bet my whole life that hes gonna kill Kreacher and Bellatrix Lestrnge the moment he see's them.
Oh, and here are some obvious similarities that I've fond between Lord Of The Rings and the whole Harry Potter series;
Gandalf=Dumbledore;
Saruman=Fudge, Quirell;
Sam=Ron;
Frodo=Harry;
The Elves=The Veela's;
Aragorn=Sirius Black;
Legolas=Lupin,
Boromir=Zacharias Smith;
Rosie (Sam's future wife)=Hermione;
Sauron=Voldemort;
Wormtounge=Wormtail, (not only the name, but both of them betrays the trust of their friend, and also both of them are behind the downfall of their friends enemy);
Shelob=Aragog;
Merry & Pippin=The Weasley twins;
Gollum (he betrays Sam & Frodo in the end)=Kreacher;
Treebeard=Whomping Willowly;
Gimly the Dwarf=Dobby;
The giant octopus thing that attacks them in the 1st movie=the giant squid;
Lady Galadriel=Ginny ( both of them have the same sort of determination, are selfless, brave and some how give you the sense of soothings and trusty worthiness);
Not only that, both LOTR & HP have dragons, goblins, giant talking spiders (oops, I've already mentioned that), and do I need to say that LOTR has a similar story lines to that of 'Harry Potter And The Philosophers Stone'? Both the bad guys are after a material which is used in making ornaments, that will give them the ultimate power, and the good guys has to protect and destroy that material so that the bad guys does'nt get hold of them and enslaves the whole world.