Here's my current thoughts on the rucks. Outside of Gawn, I don't have a clear cut answer though. Add in your personal opinion and preference to suit the rest of your team.
I think if you're looking for a keeper (which I feel you should be for the rucks each year, unless there's obvious bargains like Witts of 2017) he needs to be averaging at least 105 with the capability to go 110+. While last year nobody achieved 110, I think there's plenty of guys who can do it and given we're hoping for keepers we need to target these blokes.
Gawn - Lock
Kreuz - Looked top shelf last year and consistent, but he has always struggled to get a string of games. 2 niggles already this season. Don't mind him, but don't think he can top his 2017 season and I'd only pick him if you have cover because the risk of missing a game here and there is high.
Martin - I ended up picking him. He looks fit and ready to go back to 100+ scoring and reckon he can hit 110. Smith looks to be out of the picture and McIniery looks to be the next in line. He's not putting a lot of pressure on and if Walker and McStay are fit I can't see a 2nd ruckman being included at all this season.
Goldstein - He really scares me not picking him as he looks really good and we know he's easily capable of 115+ (and much more). But Preuss also looked good in the pre-season and we've seen massive score drops when he shares the ruck. You'd have to think Preuss is pressuring and will get a chance at some stage, and for that reason alone I couldn't pick Goldy.
Grundy - I'm actually tempted. Cox is there, but he's also their only forward now as well. Grundy is still only 23yo so still maturing and improving his game too. I think even with Cox, Grundy could still put up a 105 average this year. He might be our sides for the next 5-10 years as a top 2 ruck, but I want to see it before I pick him. If you took the risk on him, I don't mind it.
Witts - Solo ruck and worth considering. Could do better than his 95 from last year. But again, would like to see him do it first and also show he can play 22 games.
NicNat - He's gone over 105 once only which was a 114 all the way back in 2012. He's only cracked 100 3 times. He's returning from significant injury. He'll be playing limited game time for the first month and has to travel every 2nd week. Add that all up and I can't do it, but he's also the cheapest so I can understand why people would want to. I just can't see him scoring enough to carry that risk and the risk of him missing games is too high, especially when you're already burning a trade to bring him in. I feel if you took the punt on him it had to be from the start.
Then there's a whole bunch who'll be in the 90-100 bracket, which I consider just not enough for a ruck - Jacobs, McEvoy, Sinclair, Sandi, Nank, Smith, Bellchambers. Maybe you could make a case for Nank (fitness improved) and Smith (if no Stanley) to go over 100 for the year. But I'd like to see them do it first.
Just my opinion, and I think I've got a good record of judging the rucks correctly. But, if we all agreed it would be a pretty boring game!!!