Adventurous/action/RPG aspect of the game

[Radiance]

Adventurous
I will split interactivity in two subnotions:
1.)gameflow
2.)puzzles (in point-and-click adventure sense)

Gameflow interactions were part of UI were player need to click the door button, find out important piece of info on screen, break the glass to venture forth, detach cables etc. Gameflow gathers more simpler solutions to game challenges.

Puzzles are the ones orienting around difficult tasks like strange mechanisms, inventory based ones, combining bits and pieces around the environments, figuring out what are their importance, etc.

I believe puzzle items should be more scattered in level environment so that player is not always solving them on the go.
But they need to be connected in sensical way so he can create logical network which will provide solution.
This gives non-linear pace to gameflow.

There could be choices given to the player how he would like to proceed with the solved problem. With different options he could have reason to replay to get another resolve on the story. Maybe even puzzles could have more branching points.
For example, will player choose the rewarding outcome of the puzzle solution or will he sacrifice it for various reasons to proceed in different way.

Action/stealth
Encounters could have more tense design. Maybe to give player oportunity to disorganize the A.I. or even use environment design to relinquish A.I. for good or make it helpless for some time.
Using liquid trickle shader effect in HPL3 there could be salvaging aspect of the game, collecting fluid substance essential for the story elements, puzzle items or in gameplay. Fluid puddle could be used to slow down the A.I. or even disolve/vaporize it using some particle system along. This should be balanced very well. Player should weight if he will sacrifice one phial of this substance over the other one with different effect.
This way he could have different experience with the game.

RPG
As the story progress it would be nice to see character progression in terms of characterization along with playstyle. So, starting point won't be the same as later one. That way gameplay is more interesting, challenging and thrilling ride.