There are no Bon elements in Buddhist tantra in Tibet. There are a few rites, such as sang offerings, etc. that Tibetans use that ultimately derive from bon, but that are not "tantric". Many local deities were absorbed as protectors,however this phenomena is common in all Buddhist countries and is not unique to Tibetan Buddhism.
I stand corrected. Thank you for pointing this out.
On the other hand, there is also a growing interest in Anuttarayoga tantra and so on in Japan too.
And this can only come from Tibetan Buddhism as it does not exist in traditional Shingon.
While Tibetan Buddhists are generous with their tantric teachings, Japanese Mikkyo priests are not really inclined to teach it to anyone outside their own circles. Let's hope they might learn from Tibetan Buddhism...
Also I agree that in general, the elitism of the Japanese priesthood around Mikkyo in general will contribute to its eventual decline.
They are quite insistent that you will receive no serious training unless you ordain. It prevents ordinary people from engaging in any serious practice if they are not going to be taught anything unless they put on robes and go through monastic boot camp of some sort. Also one attitude I've encountered is one where the untrained and uninitiated beginners are treated like scum, being the lowest of the low. This is somehow supposed to foster humility.
This same model exists in a lot of places in Japan like on university sports teams. The juniors clean the toilets and do all the grunt work in the dorm while the seniors enjoy leisure time and educate their subordinates. When the subordinates become seniors they earn the right to leisure time and to scold and enslave the new juniors. This doesn't really lead to humility. It just leads to beginners being mistreated while they wait for the day they graduate to a higher level in the hierarchy and can seat themselves in a position of authority.
I don't think any of this will change unless Shingon or Tendai gets charismatic leadership. As it stands they don't have anyone like HHDL, the Karmapa, Ayang Rinpoche, etc.., who have international appeal and actively work outside their home territory teaching Vajrayana to everyone. Japanese Mikkyo is kept away from anyone but the initiated. However the problem is that the numbers of priests is in decline. If a generation of initiated fail to pass on their transmissions to a younger generation, it dies. All that will be left will be cool statues and some old temples.