I think many of the foes would be more goblinoid and faeish than is true in the current game. I imagine that fae and goblin tales of the middle ages would be mixed with the mythology of egypt, greece / rome, norse, and celt would be major sources of inspiration - along with the pulp tales of the current era. I can see supliments adding Arabic / Persian, Indian, and Oriental feels to the game, based strongly upon their history and mythos.
All in all, it would be a very different feel for the game. More gritty, more 'realistic' in terms of holding fast to the old legends and myths. Hmm, I think the older gothic period (Dracula, Frankenstein, etc) may also have a bit of influence over the creation of the game. They were not so old as to be cliche as of yet, after all. And consider that Computers were non-existent, TVs, etc rather uncommon - boarding on rare, and phones, automobiles, etc were well known but not yet anywhere near universal. These might also have affected the game.
I wonder. WWI was less than a generation old, and already signs were building up suggesting that WWII - or rather some type of conflict - might perhaps come before too long. I don't think many really expected a second world war, but then I think that was due more to Denial - self delution to ignore an unpleasant potential reality - than actual inability to see the signs of its potential coming. How might these have affected the game, I wonder?
Another consideration is communication and transportation. Today's game is heavily influenced, I think, by the rapid communication and transportation of modern times. 3rd party publishers known within a day how many of their customers feel about the game, and they often have up the minute understandings of what their customers are interested in seeing - if for no other reason than lurking this and similar boards. TSR lacked that resource, and to say it was even more lacking 50 years before TSR would have been an understatement. The initial game, therefore, would have changed far more slowly than typical of today - or even TSR's day. I wonder how that would have effected the game? Perhaps the game would develop its own pulp magazine (proto-Dragon?) to encourage discussion, mailins to the editor, etc as a means of gauging interest in this or that topic?