Honda Hornet 2.0

If you're not going to go on longer journeys often and want to stay in the same displacement range as your Unicorn, the Bajaj Pulsar N160 and TVS Apache RTR 160 4V will serve you well. If you want a small step up in terms of performance without sacrificing too much fuel economy, the Bajaj Pulsar N250, Honda Hornet 2.0 and Suzuki Gixxer 250 will fit the bill. You can also take a look at the KTM 160 Duke and Yamaha MT-15 because, while they have smaller displacement engines, they are liquid-cooled and offer punchy performance with great fuel economy. 

The more laidback Royal Enfield Meteor 350 and Honda CB350RS will definitely feel more special in their riding experience and fit & finish. Also, it will be significantly more expensive to buy and maintain compared to the aforementioned bikes. 

A good upgrade without sacrificing mileage from your 100cc Platina would be something in the 150-160cc space, like the Bajaj Pulsar N160, TVS Apache RTR 160 4V, Suzuki Gixxer, Honda Unicorn/SP160, or the Yamaha FZ lineup. If you want to go beyond that, the TVS Apache RTR 200 4V, Honda Hornet 2.0 or Bajaj Pulsar NS200 will fit the bill. Take a long test ride to see which option suits you best.