Yes, having the Town Portal spawning after some turns seems good, sort of like "Word of Recall" in ADOM. However, in ADOM it takes 20 turns or so and it takes you directly to town, which wouldn't fit to Diablo. 1-3 turns until the Portal appears would be good, I think.
The delay shouldn't be very long because that wouldn't fit to Diablo, too. In the original game, Town Portal was a kind of trivial escape too, because it doesn't take long to read a scroll and double-click at a position next to you.
One question left: Where should the Portal appear? Right next to you (meaning the position you are after the 2-3 turns, not at the time you read the scroll) seems good, but what if there are 8 monsters surrounding you and there is no place for the portal?
Perhaps the spell should fail in this case and the portal just shouldn't appear. This would also avoid Town Portal abuse because you had to be more careful.
Greetings,
Christian "schnaader" Schneider