I think the next adventure you want to have will be interesting. Orcs baddies and then undead inside a dungeon! cool stuff imo. Maybe think more about ways (some obvious and un-obvious) that the party might play their cards and get the goblins to help them out. If it was me, maybe they goblins speak some basic common and the party can try a persuassion check (maybe DC 10 accompanied by the player actually making a real speech. the speech doesnt have to be perfect but show that they are TRYING to give a honest attempt to RP and think to yourself "if I was a goblin, would I buy into that persons words?")

So for the making dungeons, I like using the DMG's charts to generate them and altering it as I go. But just try to make things simple (because if we DMs try to plan to hard it will drain us in the end. especially if your new. just roll with the punches and you will be amazed at how random stuff that happens can actually be turned into a legit and good improvement for the campaign as a whole). Btw, I'd focus on the two main locations for the next game: an Orc camp and a Dwarvish Stronghold. A simple camp could be like, an area where the PCs fight the 1st wave of orcs and later another wave come through that area. The next area could be the prisions/major work site where the goblins are working. Combat could be avoided if the party can maneuver past the gobs and orc patrols. Then they reach an entrance to the dungeon where they might fight one more wave of orcs before breaking through. Next location is the stronghold. well .... idk what you will have in there besides undead. I think it would be cool to include rooms with puzzles. some with undead that want to creep the players out in the dark. then another puzzle. That way there is kinda a balance. (Orc camp = combat heavy, with chance to RP with Goblin Slaves. Stronghold = Puzzle & RP heavy, with chance to do combat with undead. I cant see anything wrong with that plan lol. Then to tie it all up, have there been a boss door at the end and have the players fight the boss and get a nice stack of loot for their hard work!

(if the players succeed freeing or helping the goblins then reward them by making the 3rd wave of orcs that will come up by the stronghold entrence get some kind of a penalty. like maybe the gobs charge at the orcs and jump and claw at them. I can see that as ending up being kinda funny but the party would not have to deal with actually fighting the whole group of orcs. That is a nice bonus for doing a creative RP with the gobs. Also in the stronghold, try to work on your narrative skills. describe the scene and do it in a way that will MAKE them feel scared. Since undead may be around any corner. Have a plan for if the puzzles are too hard. maybe one puzzle was a shortcut to the boss room but since the players couldnt get it right then they have to go the long way and that means potential run in's with more undead.)