Kifaru, which takes advantage of the MOLLE/PAL system works heavily with modularity.
Don't forget about their
piggyback system.
I'd love to go with one of their smaller, flat packs like the
Marauder and then piggyback something like their
E&E.
Note another good thing about Kifaru vs other stuff like Maxpedition (although I like and have a BUNCH of Maxpedition gear), is they appear to pay a lot more attention to suspension systems.
Alas, they are a bit pricey.
-john