The "create a copy" function is probably as good as it gets.
I think it would be awesome if reddit were to blur the lines between subreddits and multireddits. Many defaults have huge lists of dedicated subreddits for stuff banned from the default in the sidebar. If the defaults were multireddits newcomers could incorporate or exclude these specific subreddits into their version of the default multi.
Of course, I'm almost certainly underestimating how impossible it would be to integrate the two like that. Still fun to consider though.
I've always wondered, do subreddits with fewer posts get boosted on the front page and in multi-reddits, or are they simply drowned out by the bigger subreddits?
I'm not sure exactly how it works, but it's definitely weighted somehow. A hot topic on a quiet subreddit is likely to end up on your front page, even if you're subscribed to many noisier communities.
http://www.reddit.com/user/Widdershiny/m/hackernews