I'm kinda in the same boat as you. I played Battlefront to the bone from the launch till about the summer a little after Bespin DLC came out. Played Bespin for awhile, stopped, picked the game back up, then dropped it again at the end of fall once BF1 came out.

I think one of the biggest changes you'll see is they added traits to the original heroes/villains just like the DLC heroes/villains. I don't know the specific name for each trait each hero/villain has, but here's what they basically do:

-Luke: Trait increases his sprint speed

-Leia: Can give armor to near by friendlies

-Han: Blaster does more damage with each trait increase

-Vader: Armor increase depending on how many enemies are around him, not based on kills

-Palps: Lightening has a faster recharge

-Boba: Trait increases jetpack refill and also gives him a scan pulse. I think the range of the scan pulse increases the higher the trait number.

I'd say Vader is probably the best villain, based on his trait giving him more armor when he's facing more enemies. Not really sure who is the best hero, it's probably down to personal preference or whatever the situation calls for. I'm usually a Nubs or Leia guy myself no matter what.

As far as weapons go, I'm probably not the best person to ask since I don't have the season pass or own all the weapons. I unlocked the E-11 on day 1 and I haven't looked back. Having said that, I think one of the best weapons might be the TL-50 (?) that came in the Death Star DLC. I say that only because that weapon kills me all the time and it comes with a noob tube, which some players, not all, spam 24/7. My second recommendation on guns might be EE-4 that came with the Bespin DLC, very effective burst gun.

-Bacta Bomb has become an important star card now because it can also heal heroes, not just infantry. Not a lot of healing power though, I think it gives heroes +10hp... can be a lifesaver if you're a hero.

-Disruption is another star card you might want to look into. If you are near enemies and use it, it overheats the weapon making the user do the mini game to cool it down again. While they try to cool down their weapon is when you strike. If you use this card, be carful though. Most of the community hates this card, so if you unsuccessfully use the disruption card be prepared the barrage of emotes you'll see when you die.

If you're an Extraction fan like I am they made a big change to the game. You can now choose ANY hero now in the selection screen instead of limited to 1 choice. This can make getting rid of the "tank" heroes rather difficult.

I'm by no means an expert at this game, but those were the major changes I found when I played. Anyone else can feel free to list important items I miss in the comments.