Heya! Thank you for reaching out to me and for including me in this project! I'd very much like to help, as I too think the idea of a combat medic has potential in League.
Before I talk about your kit, I think it's also worth looking at combat medics from other games and seeing how they achieve their playstyle: Heroes of the Storm has Lt. Morales, for example, who while being more of a traditional healer, also has some offensive power via knockback grenades, as well as a persistent healing beam that lets her focus on her own positioning. Overwatch has Moira, a character balanced between offensive power and healing, who has to deal damage in order to heal her allies. Going even further back, Team Fortress 2 has the Medic, a healing-focused class who nonetheless possesses decent offensive power on the side. In this respect, and from what I've seen in your kit, I think the goal should likely be to not only create a fighting support, but one whose combat power and utility are dependent upon each other, in order to get the full combat medic feel (which can be achievable as a Controller/Warden hybrid, and not just a utility/damage-dealer hybrid as in the above cases).
And now, with that said, here are my thoughts on your kit thus far:
- Right off the bat, the innate is my least favorite part of the kit. It's a persistent bonus that offers situational power but no gameplay whatsoever, which I think is a completely wasted opportunity on a champion who could otherwise have a host of different tools at their disposal. If you really want to keep the damage reduction effect, I suggest changing it to become adaptive: for example, have Nevon periodically adapt to the largest instance of damage he took in the past few seconds, becoming resistant to it, and have him apply that resistance to allies he uses his abilities on. That way, he'd be encouraged to tank in order to provide more tailored protection.
- Terra Mine looks excellent, and I think could be a great fit for this kind of champion. No major criticisms here.
- Progressive Remedy's surplus mechanic is interesting, though there are a few too many stacking/CDR mechanics on one ability here as well. If you want a heal that only becomes powerful if held over a duration, simply make the base cooldown very low and the cost high, while keeping the surplus mechanic. Additionally, I think it might help to make Surplus generation dependent upon Nevon's tanking ability (e.g. him tanking damage could generate Surplus stacks), rather than just pure cooldowns. Because Nevon is designed to help allies, I think it might also be better to make him automatically benefit from the heal if he casts it on an ally, just like Nunu and Mordekaiser's ally buffs.
- Endurant Bastion's basic idea of charging up to a shield that provides CC immunity is a great one, imo, and in fact I think you could streamline it to just that, and do away with the autoattack modifier. As with Progressive Remedy, I think it could be worth allowing Nevon to cast the effect onto an ally while benefiting from it, so long as the two share the same shield pool.
- Concentration to me doesn't really fit in Nevon's kit, even if it could be good on another champion. I would suggest finding an ultimate that isn't a pure personal offensive steroid on what is meant to be a Controller/Warden hybrid (neither class really focuses on raw personal damage output). In fact, it might even be worth moving Endurant Bastion up to the ult slot and having it function a bit like an Ubercharge, providing a huge shield and CC immunity to both Nevon and his target for a short period of time. Meanwhile, if you really want an on-hit effect, a long time ago I too worked on a combat medic concept, and one of the champion's abilities involved temporarily causing their attacks to sedate their targets, applying a slow that would turn into a stun if stacked enough times. This could be a way of adding offensive power in a manner that would be appropriate for a Controller/Warden.
Overall, there's a very solid base to be had in your concept, and while a few abilities could use adjustments, the core is there already, I think. Best of luck with your work on this, and please let me know how you progress on it and if I can help in any way, as I'd love to continue participating in this project!