As a Pichu player, I was very upset when he was cut. Although I was pretty sure he wouldn't make it the next time around, I played him frequently and was upset he wasn't playable. He played differently in Pikachu in the fact he took damage when he got hit, meaning your strategy had to be different than normal, especially if you played with no items on. That's what I loved about Pichu, and preferred him over Pikachu.
On topic here, I've played all three games as well. Lucas definitely deserves a spot on the roster, there's no question. Removing him would upset the fans of Lucas, both fans of him in Smash and Mother 3. As a Ness player, playing as Lucas is much different. Especially since I used to main Lucas too, but I had gravitated to Ness after a while. I personally, don't want Lucas to return, since I'm just a Porky fanboy and really want Porky to join the Series. I thought he was such a creatively written villain, one of the best even today, and his moveset would be extremely unique. Claus / Masked Man would be my next choice in a character. He would be yet another great addition to this series I believe. If Mother does get a 3rd Rep, as someone said before me, it definitely needs to get a villain for once.
On the topic of Jeff and if Mother is only getting 2 Reps, while he would have a creative moveset, he would not be one of my first choices for a Mother Rep. We don't need 2 characters from Earthbound, one of the three games in the series. Ninten would be my next choice (After Porky and Clause / Masked Man). To have a Rep from the first game of the series would be great, especially one of my favorite series. This is all just merely opinion of course, but I wanted to say something to get in on the discussion.