a Left 4 Dead 3, maybe Portal 3 and not so much a sequel but an engine update for TF2 to Source 2
Left 4 Dead shows Source pushed to its limits from what ive been told with Left 4 Dead 2 looking and playing amazingly even by modern standards and itll be fun to see Source 2 get the same treatment
Source 2 would allow the Portal series to become even more insane, experimental and fun, doesnt need any more of an explanation than that
and as much as itll change the physics system greatly (which is why CSGO wont get a Source 2 update because itll just be CS:S all over again but with even more autistic retards) the TF2 community has been through total overhauls and rebalances numerous times so having the game updated to Source 2 will increase the games longevity even more as well as allow Valve and the incredible community support to further improve and push the game past the limitations of Source and hopefully to another 10 or 15 years of life