I think that you're getting off on the right foot with altering the magic system somewhat. I am not, however, in favor of forced multiclassing of spellcasters; it tends to produce some ooky combinations. Rather, I'd suggest either having NO player character primary spellcasters (paladins and rangers are probably OK; Aragorn can heal a bit and one could argue that the ranger's or paladin's spellcasting abilities are the product of latent Dunedain racial benefits) or using the system herein:
http://enworld.cyberstreet.com/hosted/ME/SHJ/MESpellcasterSHJ.html
My ME 4th Age campaign involves one spellcasting PC using this class: Unbeknownst to the rest of the players, this character is a lesser Maia (in D&D terms, a character with one level of the half-celestial template) who eventually will blossom into his full power as he keeps at his task. That task? The shepherding of the world of Men into a re-awakening of magic into Middle-Earth. Contrary to JRRT's thematic impulse, I know, but hey, I figure it makes a good D&D campaign!
The trick involved in said task is, of course, that there are others who wish to control the destiny of the world in this emerging Age, and their motives are considerably less sanguine. Some examples:
-Urluk the orc-captain (straight theft from ColHardisson)
-A terrifying abomination, a deadly ghost-kraken corrupted servant of Ulmo (straight theft also, from the ME character competition a year or so back on these boards)
-The Blue Wizards, Alatar and Pallandro: These are a potent threat facing Middle-Earth in the emergent Age: Istari each nearly as powerful as Gandalf the Grey, firmly allied and with a growing corruption that mirrors that of Sauron and Saruman. They have gathered cults of wicked or ignorant Men to their cause in the East, far beyond Mordor, and they will not even be known to King Elessar, or the PCs, for a long time to come.
-The Court of Ardor: Another straight theft (from an ICE MERP supplement), the Court is a powerful cabal of Noldor elves led by a mighty sorceress named Ardana. In my version, the Court is made up of Elves who awoke under the stars, but far from Aman or the aid of the Valar; well, all except one, Melkor, who found them first and took them away to the Southeast, to be alone under the stars they loved. Melkor taught them to hate the Sun, as it obscured their beloved stars, and thus seduced them into pursuing a path of shrouding the entirety of Arda in Darkness, a task at which they have labored for ten thousand years. The Court is thus the most powerful of the potential adversaries to be faced by the PCs.
Others' suggestions in this thread have been excellent. One thing I would add is that you may wish to start PCs in Ithilien, under the watchful eye of Lord Faramir. They'll be familiar with the landscape (from the movies), and will always be beneath the ominous shadow of Mordor: Defeated, but not entirely beyond the grasp of evil. Later, they might be sent north to Arnor to subdue rebellious barons, or perhaps to succor the King's Messengers as they ride about their errands. Later, they might be sent to recover one of the lost Palantiri (another yoink! out of the MERP sourcebooks) and later into the Mines of Moria, which may have fallen under the grasp of an evil power. After all, the wealth of mithril to be found therein would tempt anyone (perhaps even the Blue Wizards?).