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I think asking for your viewers for subscribing you and liking your video isnt illegal if its on a good wayI know that the YouTube API has the capability to check if a certain user is subscribed to you. I was looking at doing something similar recently for a competition entry (You must subscribe to enter)
I don't know if anything exists already for what you want though. Also I worry it could be against some hidden TOS somewhere for making your users subscribe. I seem to remember reading that asking people to click like is against the TOS as you're incentivising likes etc... I'd do a bit of checking up in the YT TOS first before going any further.
Just a thought
Dave
will try to ask some Youtube Experts out there if this is legal .I totally agree with you!
In fact, if it is in line with the YT TOS then I wouldn't mind developing something like this. Seems like it would be a fun project.