That all sounds like a good idea, multiplayer would be very fun.
I do forsee some issues with online and even networked multiplayer play. I personally take about 5 to 10 minutes setting up my research tree and another 5 to start building a few ships on my first turn. this could turn out to be a very slow process if everyone had to wait for anybody who is particularly slow.
Online multiplayer is already planned with certainty and is already implemented in a basic state. An MMO version, as has been noted previously, is not going to happen however.
now your leaderboards and rankings idea could work, somebody could post things like "fastest player to 100 systems colonized" and then have a listed time, or "biggest bloodbath" (most ship-to-ship destruction in a single battle) which could list the number of ships destroyed on both sides.
While it's not really a priority, if sufficient demand is shown for such things, we might also be able to implement a more robust multiplayer "lobby" that functions as a sort of a chatroom for setting up multiplayer games. This lobby, and perhaps an accompanying scoreboard/leaderboard could incorporate "guilds" into its functionality somehow if desired. I'm not a huge fan of the idea, though, and I can't really see there being a huge demand for something like that for a 4X
I still agree that unless FO becomes a real-time strategy game (which it could, would be a pain for the programmers) actual multiplayer with multiple players on the same game would be very clunky and frustrating.
There are already a lot of 4X games that have online multiplayer - MoO2 and MoO3 being notable examples - and yes, they are always more clunky and frustrating than the single-player version; that's a price that a lot of people are willing to play for the ability to play against a real opponent. FreeOrion is being designed so as to make multiplayer as un-clunky-and-frustrating as possible, though.