The only additional HL based game I'm willing to buy OTHER THAN EPISODE 3 or possibly HL3 would be
a
HL2 RTS.
I just would never be able to resist the temptation to be able to control a squad of combine soldiers.
You would either play as like Rebels, Combine, Zombies, or Antlions.
Combine would be like lots of tech and advancements.
Rebels would be cheap with several advancements.
Zombies would be sorta expensive but also have a few biological advancements.
Antlions would be really cheap with like... almost no advancements.
OR play as GMAN. You have only 1 unit that can never die. FUCK YEAH.