I would guess that McLaren will introduce a "next generation" engine at some point, for all sorts of reasons. P14 seems like good place to start, since it's effectively their first "next generation" car.
One option would be to include lots of leading edge technology (direct injection etc).
Another option would be more or less the same design but much greater use of exotic materials. Though, unless Ricardo are already prepared for all that I wouldn't particular expect them to take that route.
However, for the type of company McLaren are I don't think they would be satisfied with "me too" upgrades to the engine - ie features that are available from competitors, no matter how bleeding edge. I can certainly understand them wanting to start with a fairly conventional engine, to reduce costs and risks for the first generation. However, they have now "earned" the right to take bigger risks, I would say - both for R&D and with customer expectations.
Re-using previous R&D is certainly not a bad thing and to my mind taking the P1 engine and turning it into a more general engine platform would make a lot of sense. If you think of it as a conventional engine with a super-powerful starter motor and super-powerful battery then the complexity is not so much in the extra hardware (though there is some) but the integration, electrical systems, control systems and testing/validation. If you conceptually slim down the P1's electric motor a bit and the battery a lot then you could have the electric motor improve low-end torque/power in general and also provide "torque fill" but not provide any extra top-end power (or IPAS).
For that you would probably need a battery capable of more powerful electric loads but not necessarily a larger one. So you would have some extra weight but probably not too much. It also wouldn't really be a hybrid.
For the P15 they could then take that and scale it back up to something close to the P1.