Although Valve will not have the time to create another source game with a "3" atthe end, and with the vast amount of bucks they're making at the moment right now, I would say it would have to be a pretty rad game. Of course they would have to implement a new source engine as running the same engine would feel the same as all the older source games (CS:S, Gmod, TF2, etc), but I wouldn't want it to be like MW3 or Overwatch, Ewww. Keeping the same feeling of gameplay while improving the mechanics is key to leaching new players. Also, as you have seen in the last decade with Team Fortress Classic and Counter-Strike, they were blocky but realistic, and as the years past, they became more cartoonish. Thus as Team Fortress progresses in the years, it will become another rendition of Overwatch but with banger graphics and effects.