Re: Plot ideas based on SPOILERS (general discussion)
As for Loki in Muspelheim and Surtur thing.
I find it unlikely to happen for couple of reasons
1) In the end of Thor1 Loki falls into wormhole, he doesn't know what's in there, what to expect and still in Avengers we see him appearing with the whole army. That means the guy can get out of pretty much any situation, find allies anywhere and make it his own way, I think it was made pretty clear in the MCU, so to me, sending him somewhere (even such hellish place as Muspelheim) is kinda letting him go.
Well, of course not completely but Odin would knew there would be possibilities for Loki to turn situation around, that's kinda danger and something you wouldnt want to happen if you punishing/imprisoning someone, there is a path to escape. Plus, simply sending him to another realm means that you can't keep a eye on him.
In Thor1 Loki said to Heimdall that he knows secret pathways between realms, even Heimdall doesn't know about, that's how he brings Frost Giants to Asgard. So sending him somewhere as a punishment is not a option.
Odin would want to keep him close.
2) Loki warning Odin about Surtur, well I simply doubt that) As I see Loki, he would better wait till half Asgard destroyed only to appear later as a true hero saving everyone, helping Odin the last minute or just figuring out what to do (that's his thing) , but I might be wrong on that one.
3) Frigga must comfort him, they're definitely having a conversation, and she assures him that he is loved and part of the family once again.
And it looks strange to me that after such a conversation Loki would be send to place like Muspelheim, imprisonment in Asgard is different kind of thing.
So it might be that after he talks to Frigga he realizes he was wrong (plus it's suggested that his rage and hate was powered up by the scepter (or gem in it). He is needed to help odin and Thor and that's his short redemption, which of course after Frigga is killed he forgets and maybe (I don't know how exactly) betrays Thor and odin's trust at the end and set to be one of the villains for the third part.