Q1: Solo Monsters
I'm not sure I get the outcry of dropping the "solo" designation. 2E had rough designations as well - the Organization entry. I'm confident that whatever direction the designers plan to take, it will include monsters designed to be a challenge by themselves, and those monsters will have that spelled out. It's been that way in almost every edition previous in one form or another. On the other hand, I did like the monster "classes" as a short-hand for their basic function, and I hope they do keep those.
What I liked about this answer was that they recognize that they're not done iterating Solo design. While I liked Solo design in 4E, there was still a lot of room to make things better. I would like to know more about 5E's action economy (if it's nailed down) in relation to monster design; one thing I felt made (some) Solos more fun was they could do a lot of stuff each turn - diversity of effects versus raw numbers.
Q2: Multiclassing
I was not a huge fan of 3E multiclassing. I was not a huge fan of 4E multi-classing. Both had systems that were relatively simple and elegant within the context of their design, but demonstrated problems that indicated they were not well thought through. What 5E needs to do in order to do well in this area is to properly iterate the system and think about/discuss the emergent consequences of each part of the system.
Q3: Cone and Line spells
I've never understood why Line spells never existed in 4E; it's a simple extension of the rules/judgement for determining line of sight. You could generally do cones the same way, but it's awkward to say the least.
Moving into the more meta area of the question, I would like to see a hybrid of the tile-based and non-mat systems. I think there's a lot of room for more of a wargame-style system that would take the best of both worlds and combine them. Something that gets rid of the awkwardness of tiles and gets rid of the ambiguity between what different people are seeing. I wouldn't be busting out rulers or anything, but I like the concept.