(Sorry if my use of Ravnica I,II,III is annoying, it's easier for me. I is Ravnica: City of Guilds block, II is Return to Ravnica Block, and III is Guilds of Ravnica block.)
I think Ravnica III will have to jump through the most hoops any block has ever had to jump through to be at the level of quality attained by Legenda, Invasion, Ravnica I/II, and Dominaria; and one of the hoops I think will be the smallest number of reprints possible.
Unlike Kaladesh, which could diverge significantly from Scars of Mirrodin and Mirrodin just as long as it had enough artifacts, Ravnica has the guilds, and the guilds dictate very specific types of cards. I would say that there will be one good reprint in each of the three sets (at the level of Rest in Peace, Remand), and then maybe two or three other reprints per set from the two previous Ravnica blocks and Invasion (we got Frenzied Tilling in Gatecrash and Invasion).
I am super excited about Ravnica III, yet there will have to be a lot of new stuff, and it's within the realm of possibility that in order for the block to be successful, it will have to be as different as possible from Ravnica I and Ravnica II, while being really fun to play, to include:
- A new cycle of dual lands, hopefully better than the shocklands, if not as busted as the fetchlands.
- A new cycle of guildmages. Ravnica I had at least one good one ([[Dimir Guildmage]], and I am not sure if any of the Ravnica II ones were good (I don't remember them being good).
- A new cycle of charms, or of cards that fit the role of the charms. It would be awesome to get guild Sagas.
- New mana rocks. The Signets were the best, and are Commander all-stars, while the Keyrunes were really Standard-only. I don't think the Invasion Cameos made it past Standard either.
- New guild commanders, and Commander, the format, is way more popular today than it was during Ravnica II, and I am sure even more so than during Ravnica I.
Oh, and the third set needs to be Return to Shadowmoor/Eventide with tons of hybrids and a return to Devotion; a mini Return to Theros, or maybe Return to Shards of Alara with the guilds forming into five shard pacts... yeah, instead of Guildpact, Shardpact.