How about adding one more thing on the poll which is "fairness". The number of provably fair slot games is not much indeed but I do think that there are still some players who prefer to play on provably fair games including slots.
I think you cannot make the slots provably fair unless they are in-house-made games like the ones bitsler and stake have. Those slots are no fun honestly because they are too simple to play though. I rather like slots from 3rd-party like the doge house, money train, sweet bonanza, and the list goes on. They are much more interesting and fun to play even though they are neither provably fair neither have the same RTP as in-house games.
For me the most important thing when playing slots is the RTP along with the minimum bet amount because some slots have insane high minimum bet like Fat Santa I remember had a high minimum bet which was late reduced. Another thing I seek is feature-buy because sometimes it is boring to chase free spins and better go directly buy them.