Learn English – Synonym for ‘reinforce/back up idea/concept’

Well. Here are a few that might be apt in specific situations. But remember, it is the situation and your comfort with the phrase that works best. Do not try too hard to not sound redundant. Nobody cares about too much redundancy in spoken english.

I'm definitely interested! = Count me in; You have my attention go on (from my boss)

Yeah, that sounds really interesting, I would love learning more about it! = Sounds pretty cool to me; Goodness me, is it true;

That's interesting! = Oh Cool...; That's fantastic; That's awesome

And how about these??

"I'll keep that in mind" or " I am going to make a mental note of it "

You may be able to reuse some terminology from software world here.

There is a standard software design pattern called Observer and its terminology may help. Quoting wikipedia:

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.

Also there is a Observable interface which may be implemented by observable objects. Quoting its description:

An observable object can have one or more observers.

[I know subject was a previous answer but I think this was too long as a comment.]