I do have to agree that there is a major need to in-game joining. Probably a good idea would be to simply set up the game to either have mid-game joins, or only at start joining.
The reason being, is that, lets say, a game of CTF or C&H is going on, people will leave mid-game, and so it would be good for people to join mid-game.
However, if it is a coop mission, then it is most likely best to have people join only at the start, and not ingame. The reason being is to make it more realistic and also allows proper co-ordination instead of a player appearing out of nowhere in the middle of a fire-fight.
OFP does have a long future, that I can be certain of, as it is really the only game which allows so much to be done to it. We have had bikes, cars, tanks, and all sorts made for OFP by the community. This is a plus other things like BF 1942 as it appears to be much much harder to modify or create addons for it. (if I am wrong, please corret me as I do not have BF 1942)