How will Iceman be portrayed?

The only thing they can do is have Maverick visit him on his death bed. There's a funeral scene in the trailer it has to be Iceman. Plot wise it'll be Maverick pondering his own mortality.

Top Gun 2 Should Have Copied Cobra Kai (And Followed Iceman)

https://screenrant.com/top-gun-2-maverick-iceman-better-cobra-kai/

Although he’s best remembered as Maverick’s wingman, Iceman is a fascinating character in his own respect and ripe for reinvention similar to Cobra Kai’s Johnny Lawrence. Val Kilmer initially had no interest in the role and had to be cajoled into taking it on, thanks to pesky contractual obligations, but no viewer could guess from his performance. Top Gun’s romantic subplot may follow Maverick and Charlie, but it is Maverick’s relationship with Iceman, moving from rivalry to outright hatred to begrudging respect and eventually brotherhood, which makes the movie a classic. A retelling from the perspective of Kilmer’s character would make for a fascinating watch, as this iteration of Top Gun: Maverick could have illustrated the cost that a lifetime spent being the sensible, level-headed teammate had on Iceman’s character.

I don't know how he'll be used other than a cameo and a dubbed voice for just a few lines.