First of all, did I recommend Re:Zero to you? It's not like I wouldn't recommend it to someone, I'm just curious.
I liked Re:Zero a lot too, but I'm also very bad at remembering stuff once I finish it. There are exceptions though. Please, don't take it amiss if I can't go much in-depth.
I'm not sure if there has been any official statement about a sequel of the anime, but as far as I know, the novel still continues. Since I don't read the novel, I can't say whether it has enough content or not for a sequel. But
@Sky probably knows much more about this, since he is more into Re:Zero than niichan or myself.
Unfortunately, I have no idea how Subaru's ability works, how he got to the other world and why he was chosen. In fact, at a certain point (will tell you later) I didn't bother with it anymore. But I agree with you, the concept is quite interesting. It's not like "I could tell you, but I won't because of [insert shitty reason]", Subaru is simply not able to. Gives the whole situation more spice.
Yeah, Elsa was amazing to watch.

It's probably my most favorite part of the anime. Too bad we didn't see more of her. On the other hand, I might disagree in regards to Petelgeuse though. I can certainly see the charm of this character, but they focused on him way too much in my opinion, to the point where I got annoyed by him and wanted to see him disappear as soon as possible. Not to mention that Rem stuff, oh no no, I don't want to remember.
I share your disappointment about Reinhard having so little screentime. But I can assure you, there is a reason behind this. I let myself get spoiled about his character, or rather about his
abilities. If you want to know it as well, then you should read the first post of
this MAL thread. It's purely about his
abilities, no story spoilers. Just to give you an idea how awesome he is:
- Sodium knowledge: He will never get salt and sugar mixed up

- Taste king: No matter how he cook, the food will come out delicious

Yes, Wilhelm is one badass mofo. He was probably the best thing to happen in the second half of Re:Zero in my view. And that flashback though, so sweet and awesome. I would definitely watch or read stories of Wilhelm's past.

As for the end of Re:Zero, I can't even remember it anymore. xD I must admit that I liked the first half of Re:Zero much more than the second one. Especially the whole (forced and dragged) drama scene between Subaru and Emilia was
unbearable to watch, so much cringe, it wasn't even funny. And since I'm also not the biggest fan of Petelgeuse, you can put one and one together. I think they handled the "mix of genres" better in the first half. It's just a personal preference though. The fight between Reinhard and Elsa was the highlight of the whole anime in regards to battle and animation. The bar was set very high, it's absolutely understandable to have high expectations, especially if you were spoiled by the gifs before. xD