Hi,
First of all i want to say that i really like Repetier Server. I have tryed Octoprint, Astroprint and for me personally Repetier-Server is the best solution with the great interface design and overall functionallites. That said sometimes i do wish there was some sort of possibility to integrate one or two additional feature into it that other printer servers have.
Octoprint with its 3rd party plugins are a pretty nice feature where you can mix and match what you want to add. Most of the plugins are really not that usefull to be honest but i find that one or two features i would love to see also here but sadly since there is no 3rd party plugin feature i guess thats currently not really an option.
I.e.: I would love to have a big ass Emergency Stop button placed somewhere on the interface and NOT having it hidden in a menu. In case of an issue i just want to hit it as fast as i need so being able to place it maybe as a dedicated button on the top bar would already be a cool feature.
I use TP Link smartplug to turn my printer off after a completed print. I have a dedicated linux script setup for that purpose which is triggered through repetier server external commands function but ideally (knowing that majority of people would not have such a plug) it would be nice if this feature would be more integrated in repetier server somehow. Possibly with the potential support of 3rd party developed plugins like the one available for this in Octoprint.
I know that currently Repetier Server is a more closed system and as such is probably also a much more consistant system (for me at least) then the alternatives out there (Octoprint) but if in the future you would consider opening up it a little bit with some possibilities for more tweaks by users that would be the cherry on top of this otherwise very fine cake for me :)
Cheers