I always thought Jim Caviezel would be a good RoboCop.
He's got the calm demeanor, the monotone voice thing down, and he'd probably enmbody the physical movement stuff pretty well. I dunno, not someone who'd cross my mind for it though.
A couple years ago I would have said Fassbender if you're going for something Weller-esque, given it's the same continuity and all. He's too overexposed by now though, love the guy as an actor but I'm just sort of sick of seeing him in everything. Lensherr's enough as far as comic-y type stuff goes.
As for the idea of Weller just doing the voice, I dunno. Might be feasible if you're not intending to take off the helmet throughout the movie, but all three originals had those moments. And he's a big part of the movement, the way Robo walks and turns was all thought up by Weller. Burke did a good job mimicking it though, and I guess it was more or less fine in the reboot. But Weller probably doesn't sound that close to how he used to anyway, you might get a Vader-in-Rogue-One situation where it's cool to have it, but it's not like he sounds identical to back then. Even though this is in the same continuity, probably best to go with a new actor providing everything, just cast him in a way where the guy looks and sounds
similar, but doesn't have to quite be identical. Just "same vibes", but leaving wiggle room to do his own thing and go a way to make it his own.
Always thought Michelle Trachtenberg shares a pretty strong resemblance to Nancy Allen facially too. Assuming they're not necessarily needing huge name movie-level people, she might be an option for Lewis?
Loving that this is a sequel too, same continuity. Not the hugest fan of Neill overall (though District 9's seriously good), but he's a visual wunderkind and he's all kinds of right for this tonally. I'd be a little worried about his ability to bring all the over-the-top snarky black humor, but given Neumeier & Miner are involved that dispels that immediately. A little bummed some other guy's re-writing their script though, that could be fine but could also potentially be a problem. Hope they have power to sign off on what he's doing with it.
Neill had better not mess too much with the suit, either, especially given it's a sequel. Some slight updates to it sure, and in a pragmatic sense making it more comfortable/workable for the actor inside, but overall it had better be damn near the iconic look:
The final suit from the reboot was...fine I guess, it was in the ballpark of faithful to the original and all, it just came off a little generic-robot-guy somehow. And weirdly given how far the tech's come the 1987 one looks way more "mechanical", like it's not just a guy in a costume. Can't put my finger on why, but it seems more than just Weller's performance, it's the bulkiness and "Detroit auto manufacturer-ness" of the costume, they'd better keep all that.