Walkthroughs are only good if the game is still new or if you`re doing live streams.I guess i would go with a series but make it unique.Ask yourself this question...what makes your content more unique than all those other gamers playing the same game as you?
Pretty much this yeah.
To add, focus on letting your personality show as you play the game. Lets Players are popular more for who they are, over what they play. Sure Markiplier is famed for his reaction to Horror, but its not the HORROR he's famous for - its HIS REACTIONS.
If the game is new, do guides. Even walkthroughs. But go fast.
If its old, experience it and find ways to enjoy the game BEYOND the game. Ask yourself - what can you bring to playing a video game that will make people want to watch it, instead of playing it themselves?
I'm currently doing an LP of 2009 Ghostbusters Game. The few comments I get aren't primarily about the game (a few are), but rather how I mock it. How I hit it with bad puns in the comments. How I talk down to the Boss's as if they were annoying kids - not taking them serious at all.
Even if a person played the game solo, they'd not have my commentary. If they watched someone else play it, it would STILL not be my commentary. Be yourself, stay unique, and show the world what you can add to what you do, to make it incredible