–I have an idea as to help explain why certain things are in various places. As these portals come into being, they gather matter from nearby (which may-or-may-not actually happen in the ‘real’ world.). This would explain as to why you find seemingly random objects around. You should also account for the placement of the portal. If a portal was in a generic forest, it would have gathered matter from inside the forest. This means you’d find basic items a lot of the time like sticks or various flowers and other natural wildlife. Due to strange nature of this place, even without sunlight entire trees still seem to grow inside at times.
–What type of ‘buildings,’ monsters and the like nearby also should be taken into account. This helps explain why there are entire spider nests inside: because it took matter (including the spiders themselves) inside of it. If you find one inside a city, there’ll be things from inside the houses, banks, grocery stores, and other things inside of it. You could even find bits of streets on the ground inside. Cities would often focus around having various zombies inside. In terms of the ‘biome’ in there, you could actually find chunks of various buildings lining the walls. This is something that you should always take into account when choosing to go into a portal or not, since a portal that spawned in a bare field is going to lack anything useful for the most part. On the bright side, it’ll also lack much danger.
–“The ground starts to rumble under you” Could be one thing the games says to hint at the portal collapsing. “You fall over as large tremors shake the surround structure.”
–‘secret’ rooms could should be visible, yet not perfectly obvious. For example, a particular wall could look just very slightly different than the rest, and have a special description: “This wall seems strangely fragile. Perhaps something is on the other side.” Or perhaps in a forested area, mini-forests could sneakily hide loot on the other side. Or, you know, triffids perhaps.
–No. No escape orb. CDDA doesn’t involve magic in the base game. I have magic in my ROA mod because for the ropic of the mod it makes sense: It often adds illogical and weird items. And frankly adds nothing even close to what could be considered actual canon to the game. You’re talking about potentially adding this to the base game, so no. Don’t have escape orbs.