by IBBoard at 28 Sep 2010, 18:54
The obvious things that I'd see in there as potential expansion are:
More bikes, attack bikes and land speeders to get a stronger fast-attack element
More devastators, plus a Predator or similar (possibly a Vindicator or Land Raider if you're rich, but I'm not such a fan of Whirlwinds) for more big punching power.
Chapter Master, Honour Guard and Librarian (probably more use for larger battles, but a potential to show off your painting and do a special model)
Overall, the Marines are a little short of options (or at least always seem to be). All you can really do a lot of the time is get one of everything to give you the choice of everything, or get more duplicates in case you want to take an army that is particularly heavy in something.