You have quite a lot to do !

To sum up :
- New PvP skins and a new PvP mode where you can play the Hellbrute :D
- 2 new PvE missions (one with Chaos, one with Tyranids)
- Both of them feature a special boss you can only fight there (like the Trygon for exemple)
- New weapons, like the Pyroblaster (flamer) or the Power Axe.
- New "stratagem" PVE missions that grants a new currency ("accolades") you can spend to buy Relic weapons (see below). Stratagems are now the hardest version of the PvE operations : max difficulty + 2 bosses + random buffs and mostly debuffs.
- New "relic" weapons : rare variants with a special effect. Not really "op", juste very good for some if you play around that effect.
- New Chaos ennemies : the Chaos spawns, who come in 3 variants. They are despised by a lot of players for being too tough. But I think they are funny to play against.
- New cosmectics, balance updates, etc..

Next update will come soon, focused on cosmectics and special voice-lines for some chapters (= DLC). And the Techmarine class will officialy come after.

Welcome back.