We only have 1 expansion, the HoT. 2nd one is coming in about 4-5 months.
1) It's not as easy as Tyria content, but it's still soloable. Hero challenges give 10 instead of 1 point, and as such are a lot harder than normal ones, so it's recommended to do them with at least 2-3 people. Every map has it's own meta event, on 2 hour reset, that requires almost full map to be successful so population there is never an issue.
2) Level is same but there's a progression through masteries. They are mostly focused on way of traveling, unlocking different merchants etc.
3) HoT added slight trinity, but that's only present in pve raids. Chronomancer (mesmer's elite) is considered best tank because it has relatively low damage compared to other classes, has a lot of blocks, and as such being able to do that in almost glass cannon gear. You can be tanky on any class with specific gear, but that's very bad for soloing pve because you won't be able to deal damage too, and active defenses are more than enough to keep you alive.
Wvw favors tankier builds.
4) Besides roaming around pve/wvw, doing story, farming HoT/LS3 maps there's not much of a new solo content. There are raids, fractals, dungeons, but they are not solo content. High tier fractals and raids (not needed but highly recommended) require ascended gear. First set is usually best to get with crafting, next ones can be gotten from said places. Trinkets are very easy to get from LS3 maps nowadays.
5) No new races, new heavy class- Revenant.