Considering FFXI is well and truly in it's Indian Summer, officially being in maintenance mode, I doubt they'll ever do it (because that means FFXI's subs will officially decline, which will be more than enough incentive for SE to finally pull the plug on Vana'diel.).

Also SE did offer something like that over a decade ago: a FFXI sub reduction back when FFXIV 1.0 was first launched (if you moved your FFXI Playonline account to the then new SE Account Management System and then linked your FFXIV 1.0 account with the FFXI one) - few bothered though, although there was a unique bonus item thrown in as a carrot for FFXIV if you did (the Hermes Shoes, which I still possess). Eventually SE just decided it wasn't worth the effort and scrapped that offer, and have never done anything like that since. So I don't see SE bringing anything like that back, alas.