We may need to re think the masternode system, how many masternodes are you aiming to (using a quarter of coins for masternodes will get you a max of 200 masternodes... that would be enough?)?
In the case of getting an extra %3, users will get 60 HAL more per year? I don't think that can cover the cost of setting up an extra box for a year (not even in VM scenarios), unless value rises a lot...
Another thing... if we can modify the reward, we may think in another feature... you said masternodes would protect the network, but since you will need a lot of coins, a Deep dive of the value (or great rise), could cause users to drop their coins. So, if reward is greater, maybe there is a way to hold the status of a masternode for a fixed term, let say you set your wallet as masternode, then your coins can't be send for x blocks (to prevent a massive drop of the amount of masternodes).
Interesting idea, which I also think would be nice to implement...especially the part about "locking" up coins used for a masternode so that they are time-locked for a certain period of time.
There definitely should be an external incentive aside from extra HAL staking. A bonus stake % would simply increase the inflation rate, and as said by others, it wouldn't be enough incentive for people to get excited about, since most crypto users are looking much more in the short term rather than long-term stake/interest.
I'm thinking there should be incentives that can be realized in the short term. EX: a trading program exclusive to masternodes runners (been done by coins before), a site for TA and charts sort of like coinigy or bitcoinwisdom, but free to masternodes runners, HAL store discounts/coupons/credit, exclusive merchandise, etc.
Furthermore, we can limit use of the rewards to block periods of time (1 week, 1 month, 1 year, etc) in which the user locks his masternode coins, and receives the reward benefits for that duration (only allowed to access trading site or TA site for the period of time, only being able to use a coupon code up to a certain date, etc).