This is the first thing I thought of. Whenever there are too many transactions, there is congestion, and that will lead to higher fees and longer times for transactions to be confirmed. I think this is one of the major flaws in Bitcoin. If bitcoin is used more as a currency, that will mean many more transactions will be done a day, and with the adoption of bitcoin increasing, this can be a problem.

I don't know anything about developing Bitcoin or cryptography, but another thing I would like, if I had a coin, would be to try and make the average confirmation time lower. Ten minutes isn't a lot of time, but as a currency, it will mean a lot if you pay for something in a store with bitcoin and you have to wait for the transaction to confirm before you leave the store. A maximum of 2-5 minutes to confirm a transaction won't be bad.