This is an amazing idea and much, much better than your last video on Spike. It's a more fair take on both the positive and negative of Spike's past portrayals, acknowledging that there is a lot to this character left unexplored. It's only right that Spike gets his due, seeing as how Twilight was promoted between seasons 3 and 4 and the remainder of the Mane 6 were promoted from student to teacher from seasons 4 to 5. The Crusaders also seemed to have been coming along, taking on different morals that addressed different aspects of their personalities, despite making seemingly little progress towards earning their cutie marks. Seeing as how Spike and the Crusaders are at a similar crossroads of trying to move up but not succeeding all that much, this seems like the perfect combination.

I like the idea that all those years of working with Twilight were actually building up to something, in that her aspirational traits and leadership skills were actually rubbing off on him, even though faulty writing in the show did a poor job of acknowledging it.

I was not a fan of "Spike at Your Service" for its reliance on a cliched plot and a forced device, but if this kind of development were to be implemented onto his character, it would actually make that particular episode look even better in retrospect. Its portrayal of Spike going through a very Crusader-esque conflict of trying to find his place by being something he isn't would be more evident in making the endgame more apparent, rather than looking like another slice-of-life plot thrown away as inconsequential if Spike's character continues to stagnate.

It's great to see a potential Spike theory consider both the positive and the negative in his past and suggest a direction that would be beneficial to his role in the show in the long run.

How ironic that I wound up thinking of an idea like this about a couple of days ago, except I didn't have Spike as the mentor, but all four at equal levels.

You mentioned how being his mentor would kill two dragged-out angles with a payoff, but there's also another reward:

Because of Princess Spike, his status as being her assistant at the roundtable feels less as an equal status and more of a throwaway. By making him a mentor, he becomes more a part of the council. Instead of relearning recycled lessons, he teaches them.

Uhhh, I think this ship may have already sailed. All signs are pointing to the CMC finally getting their cutie marks this season, and whatever mentoring Spike could do for them will probably be minimal at best in the remaining time left in the season. However, I don't think it's that bad an idea. 

The way I see it, there is absolutely nothing wrong with Spike simply remaining Twilight's assistant throughout the entire course of the series. He doesn't need any additional goal or anything else to work towards. He has plenty of time to do something else with his life if he chooses to. His service to the one who hatched him, acts as his motherly figure and encompasses nearly every single one of his hierarchy of needs is just fine. The best he can hope to do is to improve himself and grow within the role he already has. 

His situation is similar to that of Applejack's. Applejack is a character who received a lot of flak early on for being "complete" and "goalless", and most people (including Tommy Oliver) were convinced that the writers would need to develop a new desire or aspiration for her to become even remotely interesting. But this isn't true at all. Because Applejack's role within the mane six has grown significantly to the point where she is the undisputed "second-in-command" of the mane six. She has become an exceptionally competent leader, as well as a surprisingly strong fighter, and she has been further established as the crux of her family, holding them together. And all of that has happened without Applejack receiving any new goal in life. 

Since we've already had episodes dealing with Spike's sense of self-identity, what could potentially happen next is episodes dealing with his sense of priority. Episodes which examine things such as his reasoning for serving Twilight, and whether he's happier when he's indulging himself in his own selfish desires, or receiving approval from the one he strives to receive it from. And even, if the show desires to venture into this territory, the question of what will happen when Twilight and the rest of the mane six ultimately pass away and his long, dragon life forces him to go on without them. 

There's still so much potential for Spike which does not require him suddenly gaining an additional role in the show.