Dario, I don't think it is quite so simple as you make out.
At present you can start the game on M Vs everyone else on S and use the train to your advantage, which is clever but totally unfair to the S guys who are trying to push that gap, it doesn't pay to be in front or have the best strategy.
First of all it
always pays to have the best strategy, because by definition the
best strategy is the winning strategy.
Secondly, the strategy of starting on Meds when everyone else starts on Softs will not work at all circuits, it won't necessarily work on refuelling allowed races and will only allow you to take advantage of the train if there are a few slower teams running softs, because the faster guys on the softer compound will have cleared off by the time DRS is enabled.
Having said that, I think your idea of a KERS that recharges a small percentage each lap but can be deployed at any time is a good idea.
Maybe an even better solution (possibly easier to code) is the one suggested by Gilvani, limit the amount of Boost that can be employed in any one lap. Something like 10% in a lap would mean that you would be forced into tactically using Boost as the race progresses rather than 80-100% on the final lap.