It started as a novelty build, but it turns out that forcing all your allies/attachments to be unique isn't as limiting as I thought it would be:

http://ringsdb.com/decklist/view/2502/special-snowflakes-1.0

It's not quite a Highlander deck, but the large number of 1x's means that you have to be a little more flexible with your expectations for how the deck will come together. Want an ally that quests for 2? There are several in the deck, but which one you'll have available is even more at the mercy of luck than usual. To compensate, there's a full suite of draw/search effects, from Heed the Dream to Elven-light.

Nevertheless, I rather enjoy the feel of games with this kind of build: it really makes you see what you can pull off with the tools you happen to have at hand. The abundance of "Mablung showed up just in time!" moments and the like is a nice change-up from the more usual power-decking or explicitly thematic builds. Try it out, or something like it!

Yes, highlander-ish decks can work pretty well because the allies can overlap in stats, but have a different special ability each. If you're just looking for willpower or combat stats it doesn't matter if you have 3x one specific character with 2 willpower or three different ones.

I'm building similar decks now and then, and I'm going full highlander: If it has a dot, only one copy goes in the deck. This goes for attachments too. I also like to compensate with as many permanent attachments as possible, rather than events. I really need to since I don't have everything (most of the first four cycles and the two Hobbit saga boxes).

These sort of decks add more boardgame-like fun because you never know for sure what's coming, but it's probably going to be beneficial in some way

In two-player/two-handed a secondary Tactics highlander deck could work even better. They're just there to take a hit for the important guys