I've mentioned this around before, but I thought that bringing everything from L4D1 over into L4D2 was a stroke of genius.
I think any MP game sequel should do the same, but in today's environment of lesser sequel overall, Remasters without MP modes, I'd retrofit L4D into a GAAS and pimp new mods, modes, levels/campaigns etc.
Now I don't know the extent of steam workshop on consoles today, but other things exist like my aforementioned farming sim/WWE content sharing servers. Surely a L4D relaunch is even more fit in today's world?
Just don't microtransaction/in-game currency and artificially gimp/buff stats based on bs consumables. Just let me get in there and shoot inflected. For a nod to the times, let me crawl around on the floor after being knocked over, at least.