An option to select what features you want and only pay for those features which could be anywhere from like $1.99 - $14.99. It would also be nice to have an option to just purchase increased upload file size on your server to like 25 mb or add it to the level one server boost option. 8mb is too low for a server with 2 boosts IMO.
Honestly I think the best option for both Nitro members and also for you would be to add like 4mb to the upload limit for every boost the server gets so that people that give boosts can see an instant benefit from their boost instead of the big boost at the next level. The level benefits can stay the same for everything else but with the file size going up for every single boost it will encourage more members to purchase nitro as they can actually give the servers they boost something right away.