I thought of an alternative
What if all the horcruxes get transported to middle earth, with harry, ron, and hermoine and to destroy the horcruxes they must drop them into the fires of mt. doom. Now sauron finds the horcuxes first and takes up all the power of voldermot.
This ca be epic
What if during a duel, the harry potter characters including voldermot and his army are transmitted to middle earth in the second century? Voldermot joins forces with sauron and Dumbledore with Gandalf? Harry joins Frodo to get rid of the ring while Hermione and Ron go with merry and pippin...