I would like to create a blockchain network which will stay at the same level of complexity, meaning it won't take longer or need stronger hardware to mine. Is such a thing possible?
It is possible, but very stupid to do. If you keep the difficulty the same, hardware will get better and better over time (or just more of it is used to mine your altcoin) and this will cause blocks to be produced faster and faster and faster. This makes attacks much easier to take place too.