New characters you want to see in the First Class sequel?

[X-Maniac]

The other thing that keeps crossing my mind is who they show as the person who is assassinated and who will create the Sentinels.

Doubtful:

Henry Gyrich - he died in X1, with Mystique taking his place (as shown on the helicopter when she first revealed herself). Magneto says Gyrich has been dead 'for some time' and at the TV news bulletin at the end they say they have found Gyrich's body and it appears to have mauled by a bear (=Sabretooth?)

Senator Kelly. He also died in X1, turning into Senator Jelly instead! Mystique then took his place in X2.

Bolivar Trask. The head of the Department of Homeland Security in X3. Singer seems unlikely to revisit a character from The Last Stand.

Sebastian Shaw - he did create/fund some Sentinels, but died in First Class

Other possibilities:

Bolivar's son Larry Trask

Bolivar's brother Simon Trask

Bolivar's relative Donald Trask

Stephen/Steven Lang - seems like a good contender as he created Sentinels in the comics, but also the real name of the actor from Avatar and Terra Nova that people keep suggesting to play Cable

Cameron Hodge - never created Sentinels, but also a good anti-mutant villain character from the comics

Bastion - a complex story in the comics, he is the fusion of Nimrod and Mastermold, and hides in human form as he works his way up through the government. Could be adapted into film, but tricky!

William Stryker or his father - William Stryker did reactivate Nimrod in the comics, while Project Wideawake was on Stryker's computer in X2.

Wikipedia entry on various phases of Sentinels