I do this all the time, I get about halfway through converting and painting my army and I start thinking "hmmm, what should I do next?" I'm good with marines and I'd love to do a very knightly sm army with "Henry V" leading but I'm kind of drawn towards tau which is what I'd recommend to you now, the current version of 40k revolves more on the ranged aspect of the game I find and as shooty armies go I don't think you could do much better than tau tbh, st5 weapons on the basic troops? Sacre bleu!! Granted they're only bs3 (as standard) but with that many shots?!?! Do you need to hit on 3+ every time? And they can bring to bear lots of heavy fire power. But I think the thing I like most about tau is the way all the units work together rather than having one or two units that excel, you know? The scouts work their way forward, light up a target with the their marker lights, pass that info on to the big guns in the rear and order a fire mission etc. the only thing that puts me off is the god damn hideous crisis suit models but hey, that's what conversions and forgeworld are for right?