What would you think can be done to the PT to have it mirror the OT, but in reverse, a bit more? Like say, bookends for the start of TPM, and the end of ROTJ, to mark the beginning and end of the saga, or tonal changes so that episode 1 and 6 are similar in tone, 2 and 5, and 3 and 4. Or the other way round, with 1 and 4, 2 and 5, and 3 and 6.
Like, say, TPM and ANH both end similarly, with a medal ceremony, and a clebration respectively. Obviously the mirror is ther right away, but just repeating the ending is wasting having a six film saga at your disposal. ANH's ceremony is happy, and hopeful, now we know everything goes to hell in the next film, but we don't know that yet. TPM has another happy celebration at the end, but to make it more interesting, why not make it not so happy?
Like say, make it a celebration, but give a sense of foreboding, and ominousness. Sort of "hey, this is a happy ending, or what, I;m not sure actually, I think something bad is going to happen", and when ANH rolls around, you go "ah, that's better, this celebration went a lot better".
Same with AOTC and ESB, make AOTC go well, in contrast to ESB, and ROTS is already opposite in tone from ROTJ, so not much work needed there.
Basically, make each OT film have a counter in the PT. This would tie the two trilogies together better than any references to characters or items slipped in to both would, as it wouldn't be superficial, it would be a the core of both trilogies.