In HL1 I just jumped onto objects to escape from the reach of enemies.
Maybe (if it isn’t that much work) some enemies should get the ability to attack physical object, when the player stands on it directly (for example a crate). A Bullsquid for example could use its spinning attack to destroy the crate or to make the player fall from it.
Also the path finding could be optimized, so that enemies search for an alternate way to reach the player, if he is currently unreachable (for example, if the player is standing on a platform, which is only accessable by stairs behind)
It would be cool, if a few enemies (especially the Vortigaunts, soldiers and Agrunts could jump from lower hills and platforms to the ground).
And finally, maybe Hgrunts and Agrunts could being made able to climp up lower hills/walls (and maybe also ladders). This would be awesome. And the best thing is, that simply the code from the fast zombie could be used for it, which is able to climb up pipes.