The problem with the military use of bicycles is that they would be limited to being used only on roads. Bikes just can't handle anything other than hard surfaces, so I think they would be a city only unit.

As for a steampunk mod for totalwar though I have to say their is one ide aI would dearly love to see.

Airships!

Think how fantastic huge airships would be. Quick travel over both land and see on the campaign map, huge troop transportation capacity, rapid deployment deep into the heart of the enemy.

Imagine airgunship support on the battlemap. Enemy holed up in a city? No problem! Just flatten it with an aerial bombardment.

Huge enemy force got you cornered on a hill? Call in a carpet bomb!

Also it would open up a third theatre of combat for CA. They've done land, and now sea. Next could be air! And I think slow moving cannon armed zeppelins are the only way they could do this, planes are just too quick and too different for CA to properly model a combat simulation around them.

Of course airships would have to be immensely expensive to offset their huge advatages. But I think CA cuold balance them given enough time and effort.

Sadly airships are a bit of a stretch for a mod though.

Also, bicycles armed with handlebar mounted gatling guns. Awesome.