I personally don't think everybody will write every turn. Spoiling your tactics when you're still working on them, giving away precious informations talking about your important battle the very turn you won/lost it, or just those turns when nothing happens...
I think it will be more likely that people interested in writing during the game will do it once every 3-4-5 turns not to spoil their tactics immediately and just to find their muse
Of course, as you said, many players will make for stories to come more often
If you're planning turns long enough and it's ok for you, I hope you'll keep a lil' spot for me
But before deciding the map I suggest to wait for a decided number of players (as djo suggested, a crowded map seems a great idea, at least for the variety).
Oh, if i can make my last suggestion (I hope you aren't finding me intrusive, I'm just chatting and advicing friendly
) I'd tell you to at least
-consider-, on the basis of the dimension of the map also, going for easy magic research. Tales of wild spells and enchantments, enormous summons and legendary artifact are much more interesting than the simple steamrolling national army going around - and with turns eventually being long, getting to the interesting part early would boost the attention IMHO.
All from me. Best regards!