@Chamara:
Not every pokemon has a mega evolution, and the ones that do will require work for you to to get the item. Some of which you'll get from the main storyline, some of which you'll have to work for. Mewtwo and Charizard are the only pokemon who have version specific Mega Evolutions, so you'll be fine outside of that.
Global trading and fighting is still a thing. In fact there's now an additional feature called "Wonder Trading". This essentially lets you put a pokemon out for trade, and you'll be given a random pokemon someone else put up. Hayden's YouTube showed him trading a Pikachu and just randomly got an Abra.
I don't know if 3v3 fighting was removed all together; however, it wasn't used during the storyline. That being said, there's a new "horde" feature, where you'll encounter 5 of the same pokemon at the same time. Neat addition.
I'm not aware of another way to play the game yet without a 3DS, outside of having a 2DS.
Finding rare pokemon in the safari zone is back and better than ever, not worse. You can only catch so many pokemon on your version without help; however, randomly spawned safari zones exist for each and every Friend Code you have. So have 5 friends? You have 5 more SZ's to look for rare pokemon in.
In terms of challenge: Pokemon's never been challenging by just "completion". The fun you'll have will come from how exciting all these features and gameplay elements add to the current formula, the game feel from the 3D fights, and most importantly: the amount of experimentation and freedom you have with your team. If you're looking for a challenge, it'll be playing against other opponents. Pokemon is a very competitive E-Sport if you're willing to get into it all.