The problem with prequels...
is that the player may not know the exact details, but will know enough that most all of the game is spoiled. This is a shining example of that. Having played the first game, I knew almost exactly what was going to happen. It was already given that Langdon would not live and Tracy was evil. Despite this, I won't deny it was a great game, certainly better than the first. It was nice to be forced to kill monsters or get around them in creative and different ways rather than just hammering through them all and being able to die through poor choices or puzzle failures kept me on my toes. However, I still feel I couldn't get into the game well enough because of my intimate knowledge of what was already going on from the first game.
As for specific suggestions for this game, the dialogue box, in all cases, should be removed. Characters should still talk of course, but I think it might work better to display it in a similar manner as when examining items or your surroundings. Honestly, the box looks kind of goofy for a game such as this and makes it difficult to take dialogue seriously. The dialogue also moved WAY too slowly to keep my attention. This wasn't ordinarily a problem except in the case of talking to Tracy as trying to skip to display the whole text would cause her to instantly attack you. My suggestion is to have a button to skip the whole dialogue, perhaps space bar, and another, probably mouse click, to advance dialogue until a new person speaks. (For example, if Langdon was talking to Tracy, it would advance to the next line Tracy had, another click to Langdon's next line, etc.) The same principal should apply for when discovering, or being discovered by, monsters. It takes too long for the text to complete and by that time, the shock has worn off.
In the tunnel where you had to shine the flashlight on the key while avoiding putting it over the monster, I feel personally that his head was too close to flashlight. It would've been better if he had been leaning the other way near the key to force the player to be careful when grabbing it. I died several times on accident barely moving the flashlight down form it's starting position, which was more frustrating than anything, and in the chase scene, I don't think you should show the monster's position relative to yours. I think it'd be best to leave it to the imagination of the player how close he is.
As for the ending, it was completely unsurprising as I mentioned everyone who played the first game knows he would be brought back and killed, which took away a lot from the ending. Even if I hadn't known, the shadow of the man in Langdon's house was way too obvious to not be noticed. You should have placed him against a dark background to help hide him at least a little. After that, the ending would probably have been better if you had skipped straight to the video recording of Langdon. After all, I'm sure most people could easily remember Friendly saying he would be the last thing he would ever see, so that monologue was really just a waste of space and time. You need to keep your audience more engaged, and I feel that monologue just didn't do a good job.
Overall, I feel this was a very well done game that only had a few points needing real improvement. The biggest problem of all was not necessarily your fault in the fact that it was a prequel, and there is certainly nothing you can do to really fix that. I will look forward to the next chapter.