I have lurked around occasionally and when I saw this topic I had no choice but to join in the brainstorm. I like Notrium a lot and I warn you of a long post.
Multiplayer is such a hot topic and yes, I would love it too. I just wouldn't love to pay for it and I would never want to crack a Mönkkönen game.
But first on the game itself. It'll need a LOT more content. It has plenty already but far more would be great.
Imagine a massive world that would be absolutely different everytime you play it. I personally love Nethack and other roguelikes for their randomity. They are limited to a single dungeon, and just thinking about being able to play a completely new WORLD each time I play... Oh dear.
That won't be happening unless Ville will dedicate a large portion of his life to it, and we can do with much less. That much less would be, though, much more than what Notrium currently is.
So in short: Make more of everything. Make everything randomized. And most importantly, make a lot of things possible.
If someone wants completely non random adventures, Notrium has great modding capabilities just for that.
The world of Notrium currently is quite small if you compare to the game UnReal World for example. Just simple things like having to walk a day or to from the heart of a jungle to barren desert with the trees getting gradually more scarce. So as some already stated: A bigger world.
Then the items. Tons of items plus even more combinations. This is one aspect of "make a lot of things possible". Many many items of which most can be combined with each other for sometimes very interesting results. This would be one helluva job but it would add a lot to the game when testing what you can get by combining. Radiator + bio indicator = cancer gun?
Another aspect of "make a lot of things possible" would be being able to... destroy a lot of things ingame. Boulders could be busted, trees cut down (possibly for even *gasp* making your own hermit hut), whatever you can think of. (I'll list no more because then it would sound too much like UnReal World)
Oh and water + islands would be very very nice.
More critters is one thing too. Many alien creatures of different kinds. Some basic, some weird and some maybe grotesque. Adding new environments opens possibilities for new creatures and water is added... oh boy what creatures will lurk in the depths. Alien fish would be a nice add to the fungus/alien meat diet.
The place where you start the game (where your pod crashes) should be completely random too. Now don't you go thinking I don't like Mr. Hermit! Just that if you crash into his house you could consider yourself very lucky, because you could've ended up in an alien nest or the bottom of the ocean (IF water it ever added). This could be optional too: if you want to drop randomly then ok. Alien nest, hermit's hut and possibly other locations (which should be rather randomly placed ofcourse) could be chosen as crashing location.
And that inventiory screen could be split into player skills and actual items. Humans and androids should get some skills too. (Maybe the hermit can teach kung-fu?)
And finally, please don't give the player a chance to end the game very quickly. Once I played as an alien and killed those alien queens and their eggs and poof, I win. It didn't feel like victory because there were so many things yet to experience. With many different endings and all of them very hard to reach, some of them can be totally absurd if you wish. (I like fun endings)
That said about the game itself; Bear with me as I include my opinions about Multiplayer in this same post.
Multiplayer is a great thing and also a problematic thing. With things mentioned above it might not work too well. I don't know.
In my opinion a MMORGP style internet play would be horrible for Notrium (I must say I generally don't like MMO-games). A similar system like in Soldat or Battle for Wesnoth would be good.
A player can host a game, preferably he/she has already generated a random world to make things quicker (*sigh* more on this later). People can join his/her game and when the host(ess) feels like there are enough players, the game can start.
Whether everybody starts seperately, in one big group or several groups should be in the host(ess)'s decision as well as whether players can damage other players.
But as I said I don't know if such big worlds with so many destroyable objects will make the game too heavy for multiplayer, especially if 3D is added.
Complete 3D I wouldn't like. Semi-3D (like in GTA-GTA2) would be nice (terrain is simple 3D, players and aliens etc. are 2D).
A world generator/editor would be nice if implemented. Worlds could be generated, saved in files and edited with the editor. Random world generation can be a long process, I would accept that, as long as loading one isn't. It'll ease things for modders and in multiplayer there's a good chance of having a new world to explore. (I know there's an editor already, I just propose something that goes together with the other things I suggest here.)
Sorry for the very tedious and long post.
I know many things I propose will take a lot of work... but this is an ambitious post for (what I've seen so far) a very ambitious game creator.