The next Trevor McDonald

Yes, but there isn't really space after all for experiments with reporters atm, as Mark, Nina, Trevor, Nick and Katie have all been in recently in the run-up to the elections, and Andrea is assigned to the weekends, and if there is a different weekend presenter, it would usually be James Mates.
But still, I hope...

I think that they are likely to fill the evening news spot (presuming it is vacated by Mark) with someone younger who they see as having a strong future - perhaps someone like Chris Rogers. But I think that this change would have to be made gradually by first giving him more exposure on ITV1. Alternatively, ITV may try and approach one of the young, up and coming male presenters from Sky News. I doubt that they will take someone like Bill Neely or James Mates away from reporting when they are the two best-known correspondents.

This is what the Evening Standard's media diary said on the subject yesterday (Wednesday):

ITV's impressive election line-up, unveiled yesterday in Gray's Inn Road, prompts fresh talk about who will succeed Sir Trevor McDonald as the network's top news anchor later this year. Mark Austin has been heavily touted, but there will be no announcement from ITV till after the election. Some believe that news chief David Mannion is still looking at other candidates - both internally and externally. Two possible names within the ITN stable are Alastair Stewart, who is playing a prominent role in ITV's election coverage, and Bill Neely.

Well James Mates does seem to fill in for ITV1 slots quite often - he is doing the 10.30 tonight; I think its a two way battle between him and Bill for the 6.30 definitely, assuming Mark moves.