I came from the RKE and Archon so controllers are not new to me. As far as changing a 0-10v output depending on other outputs that can be done using schedules and a flow pump output but it would depend on the outputs. I was passing signals between my Archon and Hydros to begin with so I was able to add things over time. Originally I got it for the 0-10v inputs for my mixing station and liked it enough to expand it to the entire system. I got the original RKE in 2008 so I am not new to controllers. I like the gate based logic of the Hydros but that may be because I built my own circuit boards with gates back in the 70's when I built my first computer. I think is was in 1976 using a lot of gates and other TTL logic and an 8080A microprocessor. They have come a long way since then though.I think it's cool that you're geeking out on your hydros. I would encourage you to compare what you're doing with any other controller... what you're describing is achievable with all the above. I'm not trying to convince you that you made the wrong choice, I'm simply pointing out that what you're geeking out over is what many people have been doing for 10+ years.
Yes, I know that hydros can not send a 0-10v variable output based on input states. Not that they won't have this capability eventually because theyre using the community to help with ideas. But there seems to be a list of other things that have higher priority. Which indicates there are more things that the hydros can not do as well.
Keep up the outside the box thinking. And if you want to gain some more outside the box ideas, check out Neptune systems and ghl forums they're chalked full of things like your colored light warning system. But let me warn ya... you're going to be amazed!