Although I would support a revision of the current holidays, I don't think that an 'opt in / opt out' system of modules etc is a feasibla way forward.
What I feel would address the learning needs of children, as well as the working needs of parents, better would be some 'tweaking' of term and holiday lengths (to even them out throughout the year, rather than e.g. this year where some half terms are 8 weeks long and some are 5) with optional sessions available in the longer holidays for those children who benefit least / suffer most from the current system (so targeted at those who currently receive pupil premium, for example - no pupil premium paid during term-time, but such pupils receive the optional 'holiday' sessions free while other parents can pay, as they do for existing childcare).
I suppose my primary motivation is to improve outcomes for all children. So for those children whose parents are in a position to provide 'wider beneficial experiences' during the holidays - holidays, travel, sports camps, lazy days spent in homes which contain books and pencils and toys and adults who engage with the children - then a slightly shorter summer holiday would still be completely available. For those children whose parents are NOT in that position, then equivalent experiences would be available within school premises + day trips - provided both by those permanent school employees who wished to have year-round contracts [as e.g. our caretaker does] and by others who wanted holiday contracts. Some teachers might well prefer to work 2 days a week year round, while others might prefer to work 5 days a week term-time only.
The thing is, TAs, dinner ladies etc currently want term-time contracts because of the current school year and the needs of their own children which are linked to it. If the school year were to change, then the desire for such contracts may well decrease.