I’m trying to make a web app in c# that will let me get all the tweets for a certain keyword, is there anyway that I can get more returns than what the maximum limit of tweets is?
By definition, no
Joking aside, Twitter doesn’t offer full-fidelity historical data for free; to get all tweets in the past for a given keyword you’d need to use a commercial product (eg. the Search API or Historical PowerTrack at www.gnip.com/products).
However, to get all tweets for a given keyword in the future (up to a very generous maximum), you can use the statuses/filter streaming endpoint.