Wait how do they get around without mass relays. Haven't heard anything about this

I can think of a few different scenes from across science fiction that it resembles, but the one it puts me most in mind of is pretty close to home :

*The fleets arrive snip*

Anyway, I guess we'll have to wait for the Codex entry to find out what the arms are for sure - or, heck, they might never explain them; there's still bits of the Normandy's design that we don't know the purpose of - but my guess is that they're either radiators, or part of the ODSY drive system. (With some analogue on the Nexus that's too small to make out, relative to the size of the entire ship.)

As for the Nexus' partially-completed state, I've seen two explanations, and come up with two more for funzies, any of which work for me:

1. They simply couldn't build a drive system powerful enough to move it at its full size, due to either resource - couldn't scrounge up enough eezo for the drive core - or technological constraints.

2. With the promise of bountiful resources in Andromeda, they decided that it'd simply be more efficient to complete construction on the Nexus there. From everything we've seen so far, the Nexus is still capable of performing its role as expedition hub even in its current state, so there's no urgent need to get the rest built.

3. They originally planned to complete the Nexus before departing for Andromeda, but the threat of the Reapers - I think it's been hinted that some people high-up in the Initiative's hierarchy may know about them? - forced them to accelerate their timetable.

4. (This ties in with the, 'Maybe it's a relay?' discussion.) The Initiative's long-term plan for establishing a link between the Milky Way and Andromeda is indeed based on the idea of turning the Nexus into a functional long-range relay, with the Initiative's Milky Way division tasked with working on its matching pair while the colonists are in-transit. Prior to leaving, the Initiative hadn't figured out relay technology, but they were close enough to be reasonably certain that they could crack it with a few hundred years of research and development. We know there wasn't much political will amongst the Citadel races to pursue relay tech, from Matriarch Aethyta--but her dialogue also suggests that, for the Asari at least, it was an attainable-enough goal to be worth at least considering.

My theory is that the Initiative recruited Asari scientists who'd been looking into relay tech, and were frustrated with their government's lack of support for their research. Some of these scientists would stay behind in the Milky Way, to further research the technology and then ultimately start work on their side of the relay pair, while others would ship out with the expedition, to perform a similar role in Andromeda. The one thing I haven't decided yet is whether the Andromeda-bound scientists would remain awake for the voyage or not. On one level, it'd certainly be efficient for them to do so--the expedition would arrive in Andromeda with its side of the relay equation already worked out, ready to start construction on the other half of the Nexus. On another, though, I'm really not sure they'd be willing to commit to spending 600 years - a long time even for an Asari - cooped up on a starship in the intergalactic void, with no company beyond their fellow researchers, and no way out. I mean, for some scientists, that'd probably be like a dream come true--but for most, it sounds more like a nightmare.

Of course, the arrival of the Reapers fucks with this plan in all sorts of ways, but - so far as we're aware - the Initiative doesn't know about them, and so wouldn't plan around them. Heck, even if the Initiative's leadership were to get wind of their existence shortly before they were due to leave - say, Jien Garson took Shepard at her word, and didn't buy the Council's story about Sovereign - it's not like they'd cancel the one hope they have of getting the heck out of dodge, even if they had no sure way back. Anyways, my idea for Andromeda 2 is that they try to fire up the Nexus relay, but discover that there's no matching pair in the Milky Way, and so then decide to repurpose it into a big ol' mass effect slingshot, used to launch exploration craft towards other star clusters within Andromeda. (The idea being that these ships would be equipped with ODSY drives, so they could make the journey back to Heleus by themselves in a matter of months or years--the slingshot'd just cut out one half of the trip.)

(For the record, I expect precisely none of this to be in any sense accurate; I just enjoy thinking about it.  )

As for how they'd go about constructing the other half... Well, I'd imagine they have the necessary equipment/ships stored aboard the Nexus itself? It's not like construction/repair craft have to be that big, after all--in fact, generally you want them to be pretty small relative to what they're building, like the workbees in Star Trek:

*Bee snip*

...

"Though that wouldn't help much getting to and from Andromeda, you could always set up a line of 77 mass relay pairs in between and get there fairly quickly "

*Stargate snip*

(I'm actually kinda hoping this is a complete coincidence, 'cause that'd be even better.  )

And hey, I only brought it up as an idea once! 

*Derelict snips*

Oh, and finally, count me in for all the derelict stations and ships! I love that sort of thing. (Hoping we also run into some very much not derelict ships too, though. And get to talk to them!   )

BansheeOwnage , there's always so much fun stuff in your posts.

*snip*