What do you think?

I think it's an interesting question and somewhat possible. My confidence in Plarail engines re-released has risen since Harvey was released this year, even though the cars were repainted, so Duck and/or Oliver could get that same treatment the following year. The issue is that it looks like they are trying to keep the number of available engines not too high, so more existing engines will be discontinued, like what happened to Mavis, Stanley, and Dash. Looking at the current line-up of 18, I don't think anything really looks like it's ready to be withdrawn. I believe the first ten engines will be untouchable, and the only likely engine I can see getting discontinued and replaced with one of these two characters is Percy and Rocky.