Please add more slots like these. What I mean is, there should be more "target related" slots.
In others it's bit complex to understand what's going on, at least to me. Lolz. :-)
The term that you are looking for is " Progressive Bonus Round Slots " where you need to hit certain number of symbols instead of the usual 3 scatters to get the bonus feature.
There are alot of slots with those kinds of Progressive Bonus Round feature. Another option for you to try are Wild Frames, Honey rush, Tome of Madness and Moon Princess. Cherry Pop could be a great option as well but it seems that this slot is currently disabled on Roobet. Wild Swarm is also a good option for Progressive Bonus Round but unless you managed to trigger the Swarm Bonus then its not really progressive at all