The only way for a character to visit FFXIV is if it makes sense for both games (otherwise it just feels like a cheap "lol fans will eat this up because ffvii" excuse)... AND if their role in said game is not complete. Odds of Tifa appearing in this game is very low. Possibly even 0% chance.
Gilgamesh can appear in FFXIV due to FFV plot reasons. Shantotto can appear in FFXIV because of FFXI: A Shantotto Ascension plot reasons. Lightning could appear in FFXIV because of Lightning Returns reasons.