Resource gen, although could be a core mechanic, I was more so thinking along the lines of an anti-softlock feature to keep players from feeling constrained by the normal maps and forced to play creative. Thus they don't need to be all that complicated and could be as simple as, push this button X times or in dynamic rhythm to add an item to inventory. An example of a dynamic rhythm would be inputting the number for pi on one face then resetting at the end of your current limit through another face, maybe have the length determine you get bonus pieces, or that you need at least 3~4 numbers before each number gives a piece and thus incentivizing you not reset it every time.
Multiplayer for this game doesn't quite call to me because the machines you might be building are more like display pieces without practical purposes. For me building these machines would be more about having a tactile experience that deepens my understanding and thus building up a foundation for my intuition of how complex machines function(big brain speak for I just wanna learn how circuitry black magic works). I'm also not a big fan of pvp in general, though a pve in which you and fellow players fend off the spread of some corruption by walking over it would be fun.
Having said that though...I would love to see a competition/tournament mode where a challenge is put forth, and you are then required to build a singular unit to navigate/solve the problem. Different levels of difficulty for sure, but I think it'd be amazing to watch some of the creations move through a board and how each maker went about tackling the problem. The most obvious cases being mazes, but I'd love to see 'repair bots' or racing tracks and what not.
Terraria is a game that benefits most from being a social activity, where in a group work together to build up the resources to defeated each boss/zone. The division of labor and the ability to switch roles keeps you from feeling game has become stifled or tedious.
If you feel it's becoming a grind of constant gathering and fighting, you could work on exploration, fishing, base building/remodeling, or path clearing in your down time. They are technically resource gathering or base expansion too so you won't feel like you're just idling about and getting carried, but they require more dynamic gameplay(fishing is just leisure) and thus are inherently more engaging.
Should also try out gimmick builds that aren't meta but are more fun to play, I didn't really like bows nor swords so I always used yoyos and wands plus ropes for positioning.
I'd also recommend that you and your friend should check out some of the servers, just like in minecraft the servers have a whole host of custom gamemodes to experiment with. I got really into parkour at one point and it made the game so much more bearable that you can spot more spots to traverse without needing mid~late game movement gear.
If you already unlocked the mechanic you can also try playing around with wires, though last I played there wasn't much you could do with it and I'd imagine there's tons more nowadays. Making traps or killing chambers are the go to for most, but I just used it activate the elevator glitch. Don't know if you already heard but if you hammer the blocks into slopes(triangles) consecutively like a saw wave then your player model will get instantly launched just by colliding with it. I used it generously throughout my base, and that design constraint also had me remodel the base to better incorporate them.
The most fun I had in that game is...when I'm not trying to progress but just reiterating my current set up to be more aesthetic or convenient. Not optimized though, just easier to navigate, because 'optimized' would be using repeating patterns of the minimum resources required, but 'convenient' is creating visually distinct sections that help you associate everything's location more easily. My base looked more like how the Weasley hut is described than it did an impressive fortress, but it was much more fun to build and all the more endearing to inhabit throughout the gameplay.