The characters that are generally thrown around as a third Mother rep are Porky, Claus, and Ninten. Ninten is usually dismissed on the basis of another Ness semi-clone (which is kinda ironic considering who came first), so it usually comes down to Porky and Claus. Porky is probably more deserving, having appeared in two of the games, and being the main antagonist of one. Some people think he couldn't really work (size and technical issues with the spider-mech), but that's dumb. Claus IIRC is more popular, and personally I'd rather see him, but he isn't as prevalent in the series as Porky, so it's a bit of a toss up. Perhaps whatever Mother thing in development will help narrow down a clear third rep (though probably not). All that being said, chances are pretty high Mother won't get a new rep this time around, none of the remaining Mother characters are that necessary to include, and the series (while it might see a remake or rerelease or something) is fairly small and fairly dead to receive a new character. Don't get your hopes up. Mother is already represented quite sufficiently for what it is.
Oh, and sometimes characters like Paula, Kumatora, Jeff, or Poo is also brought up, but I'd say none of them have a chance.
As for Porky replacing Lucas? That's not going to happen. The only character that could replace Lucas IMO is a future protagonist (which... obviously won't happen). Even if he appeared in more than one game, at this point Porky isn't going to replace a veteran protagonist.