I can see the spike protein added to all the common cold viruses, if the capsid is big enough, so we would get a dose of capsid with every cold (less if we have a flu shot that hits). They can also use any spreadable virus that does not give long immunity (since we need new spike antigen exposure every year it seems, since that immunity wanes).
What could go wrong? The spike alone carries no problems. it is the associated virus executibles that unpack from Covid-19 after entry that do the harm.
It is a worthwhile approach but the path will be long as they will have to make sure there is no risk of 'gain of function'