The other mistake was Ronin who had a lot more to offer. But maybe they will bring him back or create a new Kree that shares similar characteristics with comic book Ronin. Movie Ronin was already a very different character from his comic book counterpart.
I feel like Marvel jumped to Thanos too soon. Ronan (done better) could have provided enough of a threat (to an Earth that until recently never even knew what was out there) with his "Universal Weapon".
With where we stand right now, Ronan could have judged the earth based on their forays into the universe (and perhaps their clumsiness and ignorance of the Infinity stones which could still be part of the broader story without going to the gauntlet yet) and decided they were unworthy and needed to be destroyed.
That existential threat, of a super-powerful alien with a super-powerful weapon, done right, could be enough to bring everybody together to fight him without going to Thanos and the Infinity Gauntlet... yet.
Then Thanos and the Infinity Gauntlet could be held in the wings as an even bigger threat that dwarfed the Ronan threat that (at the time) seemed the biggest threat Earth would ever face.
By going to Thanos so soon, they've sort of painted themselves into a corner. Without Galactus, Doom or Kang, Marvel's options are a little limited in terms of coming up with threats to match or exceed Thanos and the Infinity Gauntlet.
If Marvel could get Galactus, that would be great. "You thought Thanos was tough? Wait until you see this guy."
On one hand, maybe this will lead to smaller stories with fewer characters... and that wouldn't necessarily be a bad thing since we're getting to the point that bringing all the heroes together is becoming too much. But I think a slower build to Thanos (especially with Marvel's lack of other epic villains) and more emphasis on smaller villains who still could have posed big threats would have been wise.