I actually don't want the people to explore much. It just makes the map huge and this game is not about exploration, it is about building a Factory.
Exploration is a major part of Factorio though, big factories need vast resources, particularly with multiple depleted oil sources (nice improvement there btw). I've been playing with the Marathon mod lately and that's increased my resource hunger a huge amount, which in turn has required a much more substantial base that really feels like a massive factory, but requires much more exploration. Generating doodads and tree types (within a certain class) on the fly could alleviate a lot of save-file data relating to the natural world, but I think you (the team) may have mentioned that already.
Could using a repair pack on the floor be a good way to fix inventory stuff quick?
Some improvements I've pondered include:
Special properties in biomes, besides speed and pollution absorption. Deserts are hotter, maybe make solar panels more effective there? Reasons to visit a biome - maybe deserts naturally have richer copper sources but poorer iron sources?
I'd also like randomised solar outputs to emulate cloudy days - nothing too severe, just to add a little domestic drama. Would having pollution affect solar output be taking things too far? I like the idea, but it'll hurt. Some of the best things hurt though.
Better solar panels would be nice, I spend SO much time laying solar farms the size of a small country to feed my beacons, I'd happily pay 4x the materials for 2x the power. Maybe more?
Unique resources - something rare to get really excited about when we find it - natural gas springs, more potent coal, non-essential higher-tier metal ore. Rare materials to make special ammo? That could be a good non-essential reward, or rare&unique modules.
More rewarding biter base clearing - have some items that the biters have hoarded which the player may enjoy liberating. If you're not happy about primative aliens hoarding items, maybe spin it so you weren't the sole survivor of the ship crash - at first - other escape pods bailed from the ship and parachuted down, your buddies tried to set up their own camps but were overrun by biters.
Late combat / auto combat - I refer to Supercheese's Orbital Ion Cannon mod, it's amazing! We spend all this time researching and building a rocket silo to launch "something" into space, earning yourself a massive laser-satellite fits into the game really nicely, especially with radar-based auto-targeters. Expensive in regards to science, material cost (and power drain for target scanners), adds a useful reward for doing the rocket.
More Rocket stuff - one thing I live in hope of seeing is Factorio with the ability to slowly build a space station, space platform or colossal spaceship (also with tiles, a really big ship) and something special about space. The ability to trade with other aliens? The ability trade for rare (unobtainable) resources? Ha, this is Factorio. Why trade when we can conquer - other planets to colonise, but different in some way. The ability to fly back and forth via rocket, or ferry materials by rocket (that's going to burn through some serious rocket fuel)
Alternate/additional objectives - someone mentioned a biter queen, that's an interesting idea. A biter so strong that she's practically invincible. The challenge then is to figure that one out - take out the nearby nests one by one to weaken support, to disrupt food supply, poison with pollution until she is vincible.
You're quite right that content can be added by modders, and they're doing a great job so far, coming up with some really creative and sometimes well-balanced additions. You run the risk of doing a Mojang though - getting so far into developing a great game and just... stopping. Yes it's possible to mod things in, but searching for mods can be a harrowing task when similar mods pop up - do we go with one or the other? Weeks later we find major imbalances, or incompatibilities. How do we search for something we don't know exists? I don't expect you to add outrageous quantities of new stuff, but if there are some good ideas it would be great for everyone if they were incorporated officially, for example power-hungry deep infinite (and slow) mines or dirty mining would be a valuable addition.