If you are into modern jet fighters, I recommend looking into Lock On: Modern Air Combat. I am unsure if people still play that online as I haven't touched it in years. Also, Falcon 4.0 seems to be a never dying simulation that still has a cult-like following after all these years. As for WWII, I'd say IL-2 Sturmovik is where to go. That was a badass sim that I played a lot when it first came out. Regarding future simulations, I'm not really sure. I know Maddox (IL-2 creator) is making a Battle of Britain simulation but I haven't looked into it recently.