I don't think Chuck can cover fast passed songs again because, well their setlist consists of the 80's up-beat songs
You suggest Chuck may have any problems with singing in any style? I highly doubt that. After seeing them live 3 times this year, I'm convinced he's really developed as a singer without losing the ability to sing anything he sang in the past.
I hope they will come up with something completely new that has some references to what they had done in the past. Remember, this is a band that would always come up with something new and intriguing and I hope they won't just record a tribute to their earliest records (like FSSD with new material), or a follow up to The Gathering or The Ritual II, but a record that will create new trends in music, that will schock and charm, break grounds and please the old fans. I hope they manage to incorporate their very different backgrounds (Alex's jazz, Greg's interest in funk music- remember Flange?, Eric's black metal and classical music fascinations) to their mutual, glorious past to create something eclectic and fresh (without sounding unnatural or forced, of course). I hope Chuck uses all of his singing styles with the domination of his melodic style. I hope the production will be great and both guitarists will record a lot (I mean A LOT) of leads, that will be long, overlapping and memorable. I'm hoping for a ballad or two, for some fast songs and some slower ones and a lot of tempo changes. That's it for hopes.
Now guessing:
This will be a follow up to The Ritual for this line up, so it would be logical if they developed that style into something more modern (either something heavier and more melodic at the same time or into something closer to rock/alternative), but this probably won't happen as both Chuck and Eric seem to detest that record for unknown reasons.
Recording something in the vein of Legacy or New Order would be a huge step back IMO, given the experience they collected over the long years in music business and different styles. Still, if they were to do that I'd hope for Chuck to bring back his high pitched ends from the early days.
On the other hand picking up where they left with The Gathering would fit better to the previous line ups, not to the reunited Testament that had had their own style very remote from the modern, aggresive thrash Chuck, Eric & co. started to play later. If that did happen though, I'd expect the new record to have a lot more solos (maybe solo duels), be a bit more melodic and perhaps include a ballad or a slower song.
And my biggest hope: I hope we find out what this album is like very soon.