First Eagles offers the ability to add lots of planes and terrains, but comes with very few out of the box. The campaign is a different class from RB's, but that game had a massive budget and team by comparison.
FE is a great lite game that lets you fly and fight with little fuss, but it's not the all-encompassing obsession that RB2/3D was.
If you have CFS3, there's also the Over Flanders Field Phase 2 mod out, a total conversion that runs over 1 GB in size. It has its own limitations due to the CFS3 engine (just as attempts to put WWI planes in SFP1 and WoV/WoE had problems due to limitations), but offers an alternative take on doing a WWI sim.
I have both.