First, to define the parameters of this as well as I can, this is only in reference to Beethoven's ideas of music theory. I don't mean to ask what people from his period would have thought, rather what his own opinion would be if he was given sufficient information to see where music theory went after he left it.

1. What would Beethoven think of the directions that opera went in from Wagner to composers like Janacek, and Smetana, to composers like Korngold, and Prokofiev?

2. What would Beethoven think of the movement to bypass tonality, jump-started by Debussy, and further progessed by the likes of Scriabin, Feinberg, Roslavets, Mosolov, Stravinsky, Schoenberg, and Bartok, into the eclectic likes of composers such as Ligeti, Schnittke, Zemlinsky, Louvier, Messaien (I get the idea Beethoven would hate Messaien), Berio, Casella, Julian Carillo, etc. ?

3. Specifically, what would his opinion be on Impressionistic music? Would he find it just as weak spirited and tinkling as the composers before him?

4. What would he think about the later works of any famous orchestrator like Mahler, Mussorgsky, Dvorak, Henselt, Tchaikovsky, etc. ?

5. And of course, what would he think of the later works of the greats right after him, who wrote mostly for the piano? For example: those like Heller, Alkan, Thalberg, Liszt, Chopin, and Anton Rubinstein.

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"Your mathematics are correct, but your physics are abominable..." Einstein

It depends how you want to set up the question. I mean, if we're actually going to consider what Beethoven the actual man would think, then you'd assume that he would hate all of it. Even though Abraham Lincoln abolished slavery, what would he think of the equal rights we have in (some) Western countries today? He'd probably find it repugnant. The same can be said of anybody else in any other time. If you don't grow up with the changes, you're not likely at all to appreciate them, especially as people who do live long enough to see changes still seem incapable of leaving past generations' prejudices behind.

So, that would be my answer to the actual question, but I assume you'd find it more helpful if we constructed a fake Beethoven in our heads with his artistic ideals and then considered what a contemporary Beethoven might make of the things you listed?

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me

What would Mozart think?
What would Bach think?
What would Clara Schumann think?

I think he'd like some of the drama of Tchaikovsky's music. The kind of opera he liked was Rossini, Mozart, Weber. Maybe he might have liked some Italian opera like Verdi. Not sure he would have liked 2 or 3. Later composers for the piano it's hard to say, but he would have probably disliked most of it as some of it would have just sounded like virtuoso music to him.

You can't tell.

There are plenty of surprising opinions of famous composers about other classical composers. If we wouldn't know, could you guess that Tchaikovsky disliked Brahms? They share so much in common, and yet! Therefore you can talk, talk and talk about what Ludwig Van would like and dislike, but anything you will accomplish in this discussion will be worthless. You can't dig the ways of genius thinking.

Only I can do that.

I got to the end of that paragraph and I knew what my response would be...

But then you beat me to it!

I think you can make educated guesses, even if Beethoven might have had some surprising opinions in there somewhere as well. The composer of the Coriolanus Overture most likely would have like the Romeo and Juliet Fantasy Overture in my opinion.

I feel certain that Beethoven would have frowned on too much freedom. It takes a universal musical language in order to communicate the kinds of stories and jokes as told through Beethoven's music. He may have broken the "rules," but without them there could be no surprises.

On the other hand he might have really enjoyed the opening night of Rite of Spring.

As I already pointed out on another thread, he would have committed suicide if he lived to see that Brahms was a greater composer...

lol, I can guarrantee that he wouldn't have said Brahms was a greater composer than himself. He would have liked some pieces though I'm sure.

You can't guarantee that because I can guarantee that he would have killed himself!

Even Brahms was in awe of Beethoven. Everybody knows he copied his first symphony from Beethoven.

[Gosh - look at the time! I gotta run.]

Of course Brahms was in awe of Beethoven, which is why Brahms would kill himself after Beethoven would.

Actually, I disagree with you. In his own words, Beethoven wanted to "get at the guts" of music. He wrote a few chamber pieces that were heavily chromatic (you might even compare some of them to Liszt's Grand Concert Solo), and they ended up as a big flop with the music critics of the time. So you might say that Liszt, Chopin, Alkan, and Thalberg, were doing what he wanted to do and had been denied the right to do so, himself.

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"Your mathematics are correct, but your physics are abominable..." Einstein

As I already pointed out on another thread, he would have committed suicide if he lived to see that Brahms was a greater composer...

Of course Brahms was in awe of Beethoven, which is why Brahms would kill himself after Beethoven would.

Of after recognizing that Wagner was a better composer than he?