What's with the suddenly huge amount of "new wargame" threads?
Anyway, on topic, if you are just planning on doing it for fun, I say go for it. Many of the people who want to make new rules now seem to want to sell them, which unless extremely new and unique, won't work out. But there is no reason not to make new rules just for fun, me and my friends constantly make new complex scenarios for
40k with modified rules, just for fun.
You could look into existing rulesets for help, there is an arcive of free rules here:
http://www.freewargamesrules.co.uk/fantasy.html