Having had a brief timeout and deeper think about this, I think I have found a solution which may appeal to some (casinos and players).
How about no bonuses at the time of deposits BUT a loyalty type scheme where a cash amount (not bonus, no wagering, no restrictions) is added per calendar month, dependent of course upon the account activity BUT regardless of win/loss.
For example, if a player deposits £500 in a month at casino 'x' then the 1st of next month they get 5% cash, so in this case £25.
Could have some simple terms like total =<100 then nothing is added, max award is £100 etc
Novibet do something a little like this already but this idea would have to be based on deposits rather than wagers as I see (and experience) plenty of dead sessions these days so chance of fast busts = not much wagering. At least based on deposits (month total) players would have a guaranteed "little something" to look forward to ?
Just the bones of an idea at the moment but an idea I feel could be tweaked to work maybe???