... generally speaking I was thinking whether it can make sense to launch a new coin, let's call it Slimcoin-Plus, with the same idea of PoB and with a swap premine for all the willing Slimcoin owners
Given the number of Slimcoin addresses in the top 100 that have been active in the last couple of years (
https://chainz.cryptoid.info/slm/#!rich), it would appear that the majority of Slimcoin owners are fairly comfortable with the approach of occasionally firing up the wallet in order to collect their accrued stake. I have contemplated your suggested approach but, given the apparent fact that the majority of the community seem to be uncomplaining hodlers, I find it difficult to see any significant demand for a new Slimcoin chain, which makes the personal ROI very doubtful.
Cheers
Graham