At the end of the first half the game is "decided".  Ideally, the losing players just quit or surrender and everyone can start up a new game and enjoy a new first half.

I'm going to guess that the expansion won't affect multiplayer much unless having the expansion is mandatory for being able to play Sins online (forced to buy it) or it's so good that everyone who plays online multiplayer wants to buy it.  Otherwise it will be similar to any other mod--people could enable it and play online with it but it's unlikely that anyone would ever play it online since the game has a small online multiplayer player base.

I actually have fun in the second half too. One time I was definately was going to lose, but kept trying to persuade my opponent to attack my most fortified area. When he finally did the battle we extremely epic. Second half isn't always that bad.

I hear you.  It can be fun doing a last stand.  I suppose it is the duty of the victor to provide this fight, but it is a pretty boring stage of the game.  Defense is static..you aren't going anywhere, but it requires a fair amount of time to kill you.  It is pretty much a grind where the outcome is not in doubt.

I constantly see where somebody has 2-3 worlds left and hides his fleet behind 30 slots of tactical defenses at his HW.  It can take about 15 minutes of fleet build-up to hit a defended world, and another 15 minutes of fighting it and melting the remaining planets. The game becomes an utterly predictable slog.  I accept this...and just hope it isn't about to get worse.

Sometime this happens, many times it does not.  If there was some chance that I could be pleasantly surprised by someone who had gone on defense and used it to turn the game around for them, that would be great, but it rarely seems to work like that. 

Now, if someone knows they are losing, but an ally is doing well, then it makes sense to dig in.  Through some perverse logic of the universe, when one guy is doing well but his ally is getting hit hard, they often quit rather than try and hold out for help to arrive.  I guess that is the problem....some people throw in the towel really quickly before the game is decided and others refuse to quit long after...and do to the pace of the game and the nature of static defenses, it takes a while to slowly push them off the map.

I'm just hoping we can have more of the exciting first stage and that new defensive options don't make the second stage drag out even longer where the game becomes a predictable grind.

That is kind of an interesting idea.  On the flip side, the guy who is winning doesn't really need a culture boost because they have more planets and more money to build more culture stations.  Unless the thing that got boosted was the actual rate of culture change.  I don't think it would make it in though.  People hate getting knocked out by culture.  The rate of allegiance decay got tuned down a few patches ago, if memory serves, for this reason.

Maybe one of the super stations will have an additional culture effect, or culture amplification effect for the local planet?