Maybe Kyoto Shogi? It has some funny/remarkable changes from Shogi:
"In kyoto, there is no promotion: at the conclusion of every move the piece that moved must be reversed."
It is also played on 5x5, but with 2x5 pieces. Here is the site: http://www.drjochum.de/ (click on the link)
I haven't played it, I leave it to the expert Shogi players to judge :) but looks fun to me.

I found some larger Shogi Variants but to me they look to slow for a turn-based site.