Travelling on easyJet and the seat tickets are cheaper than the cost of putting a bag in the hold.
Can I book a seat and put the suitcase on it?
Short answer, NO. Because EasyJet does not describe this as a possibility anywhere publicly. Also, since it would be difficult to secure a standard suitcase, the crew should object.
However, YES, if the 'suitcase' is actually a cello. This is specifically described in EasyJet's baggage terms: Musical Instruments