Here's a left-field option - Christian Tuipulotu. Noticeably bigger than Walker (Tuipulotu - ht: 186 cms, weight: 106 kgs) - he's as big as a decent sized second rower. Obviously quick enough to play in the outside backs, so would add zip in the middle third.
It will come down to whether or not Tuipulotu wants to make a name for himself sooner rather than later, and put his hand up for this new-style 14 role or not. His weakness would be hands and passing game - this new 14 role also acts as a pivot/ball-distributor in the middle of the field. In that respect, Lawton is better.
Clubs may even start recruiting for 'specialist' 14's.
Maybe have Tuipulotu as a '15' - a cover for the backs on the bench, and a cover for second-row as well - and keep Lawton as the 14. Our emerging depth is probably part of the reason that Walker is leaving - he knew that he didn't have a lot of hope cracking a centre spot with Manly again (about 4 in front of him, even with Suli leaving), which is obviously what he wanted.