Okay, so let me address your comments from top to bottom.
-Tirria will be able to become a futa in the future if that's what the player desires ^^
-Futa on MC content will become available in the future, but it will always be optional
-Charlie is actually a side character, who didn't even exist before Beth's story was fleshed out ^^; all main characters (which includes most of the cast) will have futa scenes independent of other characters.
-Rei will 100% be involved in future story scenes. Please look forward to them ^^