Many years back - long before the first mod release - I suggested that when aliens appear, as well as having just a zap of green electricity, there could be a ‘warp’ effect, to create the impression of a spacetime rift opening. Similar to the distortion effect when a Strider charges its big energy cannon in Half-Life 2. Also, in HL2 Episode 2, when the the anti-Strider bombs are teleported to you, Valve had the teleportation effect be a similar sort of black hole distortion.
At the time, I think the idea was dismissed as being potentially too graphics intensive, that a distortion effect during action scenes with explosions could cause a big framerate hit. Especially if lots of aliens teleported in. (This was back in 2009 or something, heh.) Personally, I never thought that was likely to be a problem, games like FEAR had done distortion warp effects every time a grenade or barrel exploded.
Recently, the in-development Source mod ‘Peer Review’ was announced - a Source engine remake of Half-Life Decay - and their first video shows that they’ve implemented a distortion effect whenever an alien teleports in. It’s quick and brief and subtle, but it looks pretty cool. I’ve linked to a moment a few seconds before some aliens teleport in. (There’s plenty of similar moments before and after as well.)
When I saw that video, I immediately thought ‘Now THAT’S what I’m talking about! After all these years, someone has finally done it!’
Also, the latest video from the developers of Operation Black Mesa / Guard Duty (the in-progress remakes of Opposing Force and Blue Shift) shows that they’ve implemented distortion effects for the muzzleflash of every gun. Maybe that’s a bit excessive, but it illustrates my point. Nowadays, in 2019, distortion-effects can be easily handled by gaming PCs, they aren’t going to cause big framerate hits.
Back in 1998, the best Valve could do was a green explosion sprite and some green electricity. But things have moved on since them, I think Black Mesa could do much better, without ruining the Half-Life feel.
The ‘Peer Review’ mod has already demonstrated that it looks good asthetically, and it still resembles the classic Half-Life teleport effect, the distortion is simply an enhancement. It also makes sense thematically. The resonance cascade causes a ‘dimensional breach’, Marc Laidlaw’s Half-Life story synopsis talked of ‘spacetime shattering’ and ‘rifts’ opening everywhere. And Valve themselves did a warp effect for teleportation in HL2 Episode Two. So rather than just a green flash and electricity, lets see some space warps!