Squeal Cat
I Love YTtalk
It might be a possibility that your idea about tagging works....I posted about this not too long ago. I have a video that is getting 1500 views a day, 3/4 of which come from 'suggested video'. When I look through the analytics, all the videos that viewers are coming from are subject related videos, and the title contains similar words.
I wish I knew how to get more of my videos to be suggested, but one way to try would be to look for popular videos in your niche, and then post a video using similar keywords and title. (Tags too I guess but not sure how that works since I haven't analysed down that far yet).
Imagine you are searching for something and have just watched a popular video giving you some information.. as a creator if you want to piggy back off the suggested video of that popular video, think of a title that will get people to click on yours next, instead of the other suggested video listed on the side.
Videos suggested by YouTube are there because you have a certain amount of watch time (don't mistake this for audience retention). It then becomes suggested and based on those results, it either will become further promoted if it meets certain criteria or it gets discarded by the system. There's no much you can do to replicate the success other than trying to increase your watch time across the channel.
I think this theory really comes close to the reality. The two important things are indeed audience retention and total watch time and they determine the rankings. And I think that should increase the suggested videos
. About that discarding, I like to believe that. I hope they don't do that to my videos...

A bit disappointed at the end 
) and all I see is dumb crap from above. I am disappointed in some of my viewers who can't even bother to watch 5-10-15 minute Doom videos.