I just finished watching season 1 of TAS and some of those storylines would work with future First Class sequeals:
the beginning of the Mutant Regristration Act
Anti-mutant hysteria & groups i.e. Friends of Humanity
intro. government characters like Trask, (younger) Robert kelly, or Gen. Lang
Genosha w/mutant slaves
Sentinels
the Brotherhood should be a known threat
Prof. X should be speaking out on mutant affairs/peace
Team constiting of Cyclops, Jean, Beast, Storm, Banshee, Havok
Beast leaving to work within the government
possibly the death of an X-Man
*Possibly Days of future past storyline(w/a twist), it may be farfetched, cause these films are grounded in reality, but it would be cool to intro. Bishop & Forge, who travels back in time to stop Apocalyspe(the cause of his futue),the assination of a political figure, & uprising in sentinels + it would be cool to see Mags/the X-Men team up against a powerful mutant*