Timid would be a better nature if you're using a Specs Trick/Switch set (and Scarf would generally be better for that).e 4
Modest is arguably fine for an offensive set. Just check the speed tiers to see the difference in what you outspeed, watch out for those particular threats, and be aware. Rotom's base speed isn't extremely high (which is why the supporty sets usually run Scarf), so there will be some threats that will wreck you. (For example, Timid Scarftom can outspeed Scarfed Dnite, but I don't think Modest Scarftom can).
If you're gonna run modest, I suppose I'd suggest the typical 4HP/252spatk/252speed set for special sweepers and use him as a hit and run attacker. At that point it's really a matter of picking out what form you like and relying on big fat STAB hits. For example, Rotom-W with Tbolt/Hydro Pump/WoW/Volt Switch or Pain Split.
The problem with Volt Switch is that for switching moves you generally want to either be as slow as possible (so that whatever's switching out can tank the hit) or as fast as possible (so that you can ping for a bit of damage and switch out to a resist before the attack lands). For a set that includes Volt Switch, I'm not 100% convinced that anything less than max speed is particularly worth using unless you do a bit of research and determine that partial investment in speed lets you outspeed specific major threats.