I personally want another L4D game. I mean its fun playing L4D2 online with people and I mean the modding community does a lot but there are definitely certain limitations within the game that aren't exactly fixable by mods, at least none that I have found.
Never really played half life or half life 2 though, closest I came was a half life level remake done in Gary's mod.
I loved L4D 1, played it a ton. L4D 2 I didn't really get into much. Fortnite is the new L4D, although I only started playing the free Battle Royale I haven't bought it but considering it to try the L4D mode.
My love for L4D2 came with the fact that I am slightly younger so by time I got the two games, almost nobody was on L4D online and instead all went to the second one and played the original maps (old maps, new mechanics)
If I ever get the time to sit down and play I will check out fortnite.