Here are some ideas I’ve had for Xen. Obviously the Black Mesa team has their own ideas and they’ve made too much progress to implement these ideas; these ideas are just for fun:
1)Instead of the little cave underneath the first map activating a Xen teleporter, it actually starts the island moving. The island connects to the bottom of a larger island, with the spikes plugging into holes in the bottom of that island, and the floating platforms changing their movement pattern to allow access to that island. That island would be the second map.
- The first chapter, Xen, would be longer, and would consist of multiple maps. Some of them would be loosely based on the first half of Interloper, the pre-factory part, with us seeing lots of Xen creatures in their natural environment, including Gargantuas, Tentacles, and Snarks. Non-sapient Xen creatures would be much less hostile to Gordon on Xen, not being scared out of their minds by being teleported into a completely alien environment. They would still target Gordon if they perceive him as a threat or as a food source, though. No teleporters would feature in the chapter, nor any grunts or controllers, and Vortigaunts would be kept to a minimum. At several points throughout this chapter, Vorts and Xen creatures can be seen teleporting away - at some points, this even includes chunks of the Xen landscape. At one point, one of the sting rays (or are they boids? I can’t remember) is ridden through empty space in Xen to the next map. The chapter as a whole would focus on the organic nature of the Xen islands, with a lot of toxic pools of stomach acid, organically moving land structures like others have suggested, and so on. Xen Flora is actually part of a superorganism that makes up these Xen islands. There would be one particular island with a loop of several Xen trampolines in a circle that project towards each other, getting Xen creatures - and potentially the player - stuck in an inescapable loop, forced to either escape and die of fall damage or to not escape, starve, and become absorbed. (The ragdolls of many deceased Xen creatures, as well as several living ones, would be stuck in this loop to warn the player not to step on the Xen trampolines in question.)
3)At the end of the Xen chapter, Gordon is stuck on the corner of a new island, thus forcing him to walk into a half-functional - and human-originated - trap. He is pulled into an abandoned Black Mesa outpost for HEV scientists studying Xen. This would be a new chapter inserted between Xen and Gonarch’s Lair - Research Station. The station was evidently abandoned prior to the Resonance Cascade thanks to an attack by Vorts and Grunts, compounding the questionable ethics of the “in-the-know” Black Mesa scientists - they knew that there was a dangerous civilization residing in Xen that was openly hostile towards humans. The chapter as a whole would be akin to Questionable Ethics storywise, and akin to Blast Pit gameplaywise - Gordon’s goal in the chapter is to get enough of the station’s systems online to activate the central teleporter and its control panel. In doing so, he must fight his way through several groups of watch-Vorts, and eventually through the Grunt and Controller reinforcements that they call in. It would be implied and/or stated that this research station was actually the location that Gordon was supposed to be teleported to in the first place, but that the Alien Controller attack messed with the configuration process. Once Gordon has gotten the teleporter and its control panel working, he must use the control panel to select a destination. Some destinations are inevitable death, but by viewing the thumbnail and reading the description, Gordon should be able to tell which ones these are. Others are small battles, health/energy/ammo deposits, or simple pointlessness. However, a player who remembers his goal - kill the powerful entity keeping the Resonance Cascade open - will read the description of Gonarch’s Lair and assume that this is the entity he was sent for. Gordon sets the teleporter to Gonarch’s Lair and enters it.
4)Whenever Gordon uses the Research Station teleporter to teleport somewhere, he appears in a matching teleporter. After some short period of time, the teleporter recharges, allowing Gordon to return to the Research Station. However, when Gordon teleports to Gonarch’s Lair, the Gonarch arrives before the teleporter can recharge, and begins the fight with Gordon by throwing the teleporter off the edge of the island. The Gonarch’s Lair chapter as a whole would feature a sort of “headcrab drainage system” - headcrabs and baby headcrabs are all being funneled, using small holes Gordon can’t get into, to one spot at the bottom of the island. The visual style of the chapter suggests an environment that was once natural, but has been modified by the Nihilanth - the headcrab drainage system in particular was put in place by his forces. Think the Combine - a mix of organic and mechanical elements - but with more of a focus on the organic elements, with the mechanical elements clearly being quick-fix additions, possibly made with stolen Combine technology. When Gordon defeats the Gonarch, he finally gains access to this bottom bit of the map that all of the headcrabs and baby headcrabs are being funneled to. It’s a little Xen trampoline, with some mechanical alterations made to it, presumably by the Nihilanth. It projects all of the headcrabs and baby headcrabs that touch it far, far out into the distance, out of the map. Gordon steps on it and is projected into the next map, the first map of Interloper, beginning in a Vortigaunt-operated headcrab collection area.
I’ve got to go, so I’ll post my ideas for the second half of Xen later today. 
