It'd be very weird to do a mix of old and new mechanics IMO. And they won't bring them all back so I hope we're getting ten new mechanics.
That's if they're returning to the discrete 4-3-3 Guild model, which I was skeptical of at first, but is looking more and more likely
I really hope they return to some underused mechanics and rework some of the messier or overused ones in RTR.
Like Forecast was interesting but it really didn't do anything. There's quite a bit of design space for it.
Hellbent was meh, it felt like threshold. R/B should have something a bit cooler.
I looourrrrrrve Haunt but it mechanically it was messy and made for some awkward play, I can't see Haunt coming back at all.
Flavorfully Graft can't really exist cause Viggy and the Bunch are gone, though I wouldn't mind at all if they just slapped Poliferate on Simic and called it a day. I love that mechanic and I always want more.
Bloodthirst was already done again in M12, would be too soon to come back to that. Then again, RTR design probably happened around when M12 was released so maybe we'll see that too.
Dredge, well... I really doubt WoTC wants to make any more dredge cards that already exist. Maybe if they redid the mechanic somehow that wasn't dredge, but was like dredge, then we'd see something.
Replicate was cool and again still more design space, but mechanically it was kinda ho hum.
Transmute was cool, but ever find it strange that Dimir was schizophrenic? Like it had this Mill Theme that really didn't play well with its Guild's mechanic.
Convoke was pretty cool, very flavorful and I wouldn't mind seeing it return at all.
Radiance honestly while flavorful felt like Hellbent in meh. Unless they do something really cool with it, I would hope Boros got something cooler than that.
So yeah, I'm with you, I'd like to see a mix of old and new mechanics. I've got faith in WoTC though because the past 3 sets have been knockouts in terms of Flavor/Mechanic for a set, I think they'll do Ravnica justice.
But yeah, if anything I'd rather see 10 new mechanics. The old 10 felt like Old Ravnica, before the Guildpact broke. Now they have to come up with something in a new world.
I mean if you look at Scars block as a template, the only returning mechanic was Imprint. Affinity was retooled to metal craft. With that in mind, I think they'll go with 10 new mechanics, possibly with a few that are "fixed" versions of previous ones.