So we know that there will be more games after this and my question is what does everyone think might happen in future games.

Like maybe the next game will take place in the towers, and we know the main enemies of the first two games focus on a single type of animal for the enemies, what do you think the next enemies will be lizards, bulls, horses?  What kind of scenes or new spicy content do you think it might have. I'd lie to talk about ideas like that.

Also what is behind that door in the boar alleyway when you go down the stairs 

I don't want to dive too much into speculation land, but I have some ideas.  

1. I agree that the next game will likely be in one of the towers since the quest in this game was to gain access to a tower.  For the enemy type?   Could be Dobermans since we see them slumming in the gutter in GT, but I could easily see bitshift going in a different direction.  Each game has its own color too, neon pink for CS and neon green for GT.  I don't know what to expect in the next game, but maybe neon blue for the tower.

2. We know there is a mastermind behind the events in the first two games, having provided the ear clips to Diezel and probably the Dragon's Breath to Mahir (this is not explicitly stated, but the Dragon's Breath is a new development for the hyenas so it's likely they got it from the same source since Mahir is a cat's paw for the mastermind).

3. I'd wager that Scratch is also a (likely unwitting) cat's paw for the mastermind since it's through him that Mezz gets sent to both the island and the gutter which were both set-ups.

4. We don't know what the mastermind's goals are, but we know they have some mind control power at their disposal, at least enough to control Mahir.   The mastermind is a 'fan' of Mezz, but we don't know what that means and whether messing with our protagonist is a primary or secondary objective. 

I have some other much more speculative ideas about what's actually going on, but they have limited support from the actual text, so I am reluctant to share them just yet, we are only on game 2 of a planned 5 so it's still really early in the story.

looking back on how many bad endings have him also captured you're most likely right, but now I'm wondering if he might have a bigger role in the next game, like maybe as a companion character

One new thing I noticed on my latest play-through:

Outside Mahir's place there is some graffiti that reads "Aut Furit, aut Lachrimat, quem non Fortuna beavit." which means, "He whom Fortune  has not blessed either rages or weeps."  It's the epigram for a 17th century collection of instrumental music called "Lachrimae" (literally "Tears") by John Dowland.  This is referenced one additional time, at the end of the hero-path (fighting your way to Khazeem), when Mahir removes the conditioning he uses the trigger word "fiLachrimae"  - now my Latin sucks, but this looks like fi Lachrimae, the present imperative conjugation of factus - to become - and Lachrimae - Tears.  So he orders Mezz to "become tears", clearly a reference to the earlier phrase.  

What does it mean?  I have no idea, exactly.  Maybe just that Mahir is a music and Latin nerd, but it probably has longer-reaching implications.  I'll have to think about it more.

Notably, John Dowland has another song that was reworked from a piece in Lachrimae called "Flow my tears" which is referenced in the book (and title) of the Philip K. Dick novel "Flow my Tears, the Policeman said".    While this novel doesn't really relate to Cruel Serenade directly, I would observe that the setting of the game is pretty clearly inspired by Philip K. Dick's broader body of work, especially his use of unreliable narrators, memory alteration and generally just being trippy.  And writing cyberpunk dystopias, of course.