Full Walkthrough on Youtube

This is the first time I've heard of the Timesplitters campaign. My review won't be based on how close it sticks to the lore as I know none of it. Instead, it will focus on the factors of replayability and some comparisons to official Valve maps.

Helpful Items
One overall suggestion that'd help improve the campaign is leaving throwables, melee, and other healing items scattered throughout the maps. Very early on, there is one melee weapon that spawned. It was a bit tedious to consistently run out of ammo while dealing with the hordes in Chapter 1's alarm triggering event. Throughout Chapter 1, there wasn't much of vomit jars to give players and bots time to reload.

In Chapter 1's ending saferoom and Chapter 2's beginning saferoom, there were none to be found. There's only healing items with Tier 1 and 2 weapons. I appreciate that there's Tier 1 weapons being handed out as I have a mod that allows you to dual wild two Tier 2 weapons and a Tier 1/melee weapon of choice. However, there were no throwables again to get ready for larger hordes.

Make Items Stand Out and Finale Placement
As in the video, some people like myself might play without the glowing outlines to indicate where items are.

I noticed this setback stood out the most in Chapter 2's finale. The lighting assisted players by indicating weapon cabinets are unlockable is absent as there's only one ammo pile. I had trouble finding other items scattered around the map as the ammo pile was the only visually distinguished item by being set on a table. It took me a few minutes to wander in a circle, "Oh! The pile was here all long." Perhaps setting the button in a closer proximity to the ammo pile, or putting the ammo pile next to the button with another light source would help?

There are some custom and official Valve campaigns that chooses to give players their items while inside of the saferoom (gauntlet runs) without providing more throughout the level, or chooses to hand them out at both (saferoom and finale center). Keep this in mind when setting up future finales as the players do need assistance in enduring hordes in Expert and Advanced modes. Forcing players to rely on their weapons and one ammo pile can make covering teammates more difficult than challenging.

Melee
Not much of melee weapons are present on the map. When dealing with large hordes, there should be some melee weapons available to grab.

Conclusion
My TL;DR feedback is this:
- Spawn more melee weapons
- Make items stand out in finale for better accessibility
- Spawn more throwables

Be sure to reward players if they want to explore the maps. I've encountered many campaigns where the scenery is extremely detailed but had a huge lack of nooks and crannies for players to grab extra items/weapons if their inventory is low. To avoid this, place a spawn with at least 4 items to share for the entire party.

Example: Going into another room might have a spare pipe bomb. In another room that's far from the first room, spare some vomit jars. Perhaps if someone didn't follow the correct path everyone else is passing through, maybe there's a medkit.

Well done on this campaign! I hope to see more from you in the future. The concept of time travel is frequently touched upon but few in executing well.