Makrock
Level 100 is probably too high, I'd even go so far as to suggest a cap at level 50. It's long enough for newbies to learn the game enough to decide if they want to sub or not, which if I'm not mistaken is the whole point of a free trial.
That would basically be a throwback to the past. If I remember correctly, there was a limited free trial just like that, back in 2006. Considering that the game did bankrupt not too long afterwards, it probably didn't help.
Why not go all F2P way? Make buyable armor dyes, new tattoos and haircuts, cosmetic armor designs, mektoub bodypaints / saddle designs, buyable player houses. It worked for other games... it doesn't have to be unbalancing.