The idea is good and is something I would find useful, however there are many questions that I have regarding the actual functionality which would either make it useful or not:

1. Are you going to limit it to just a few exchanges?
2. Will you be using your central API key just to pull the information you need for calculation?
3. Will the user have to program his or her own API keys into the program?
4. Will you just give the answer and then the user has to do the transaction on his or her own outside of the program or will the program do the transactions on behalf of the user with full trading API keys of the user?
5. Will the user need to (a) fund the exchange accounts with all relevant coins including BTC or (b) will the coins still need to be moved to the relevant exchange?
6. If 5 is (b) how will you account for block times and confirmation times in the movement of coins between exchanges?
7. Will coins be moved between buy and sell transactions or will transactions be simultaneous on both exchanges utilizing pre-funded coins, with settlement end of day or after a period of time?
8. How will you account for transaction fees as sometimes there is arbitrage but transaction fees are higher than the gain. Therefore transaction fees will negate arbitrage up to a certain threshold after which it becomes profitable. The frequency of settlement also plays a role here.
9. How will you take into account blockchains that are stuck or coins that are under maintenance on exchanges? Many times arbitrage develops because of an issue with the coin on the one exchange and market forces cannot keep things in equilibrium because of the issue. This could create a false arbitrage result and the coins could actually get stuck in transit.

I would appreciate your answers to these questions to understand your project better.