I think it is a great idea.
The differences from Dharmic religion to Dharmic religion are not as great, as they are from Abrahamic religion to Abrahmic religion. They share many of the same doctrines, and they only differ on the nature of ultimate reality or ontology, but even these differences are semantical.
Some of the common doctrines between the Dharmic religions:
1) The doctrines of karma, samsara, reincarnation and lokas: They agree on the law of karma, they all believe the soul moves up and down from loka to loka in samsara.
2) The doctrine of spiritual evolution: They agree that the subtle or transmigrating body evolves from simple organisms to complex organisms, from plants, to animals, to humans, gods
3) The yamas and niyamas: They all agree on the same moral precepts
4) The doctrine of avidya: They all agree that ignorance of our true nature is the cause of suffering, and also agree that suffering is a universal condition
5) The same cosmology: They all all agree the universe is an eternal never ending cycle and agree on the same subtle elements and gross elements
6) Returning to a pure nature: They all agree that the soul has a pure nature, Brahma nature, Buddha nature or God nature and through practice we must restore to that nature
7) The same epistemology: They all generally agree with perception, inference and testimony as the means of knowledge
8) Moksha: They all agree liberation from samsara or cycle of birth and rebirth is the goal of life
9) They share gods, historical characters: In Buddhism Indra, Buddha, Saraswati and many other Hindu deities are shared. In Jatakas one of Buddhas past life is Lord Rama and Buddha is born in the same solar dynasty as Lord Rama; in Hinduism Buddha is an Avatar of Lord Vishnu; in Jainism Rsibha is the first Tirthakara and a brother of Bharat, in Hinduism Rsibha is a Vedic Sage and Bharata dynasty is from which the Mahabharata follows
10) Yoga: They all agree some of of Yoga practice, using a combination of contemplation, postures, breathing, meditation practices. Most of these practices were developed during the Tantra period, which was interdharmic, and they developed concurrently with each other
11) Samkhya: They all agree in the distinction between consciousness and matter
12) Detachment: They all agree that we must detach from sensory and wordly pleasures and cease all desires
The similarities are so many that I consider myself in very close kinship with all Dharmic religions and have tried to learn about all of them, even thought the tradition I am more focused in is Hinduism. However, I accept their scriptures as pramana, as much as I accept Hindu scriptures. Sometimes what I can learn in Buddhist and Jain scriptures, I don't find in Hindu scriptures and vis versa.
The differences to me are mere semantical or attitudtional
1) No Self -- Buddhism does accept the existence of a momentary self, which is the transmigrating entity that moves from body to body. It calls this a false entity which is made of an aggregate of thoughts, feelings, perceptions, elements etc. In Hinduism we call this the self the false self too(ahamkara) which similarly is an aggregate of the thoughts, feelings and elements. The difference being Buddhism does not accept beyond this a permanent self, yet they do accept some sort of fundamental nature(mahanirvana) or tataghata or dharamkaya. In Mahayana Buddhism, the idea of a true universal self or pure mind is also admitted.
2) No God --- Buddhism remains agnostic about God, but it does accept the existence of deities like Lord Brahma and minor gods. Jainism does in fact admit of gods, but gods as plural, it does not believe there is a single god that created this universe, but rather all souls can realise God-nature and become God themselves. In Hinduism, the same ideas are shared, all sages, gods can realise their Brahma-nature. What "Brahman" is whether personal or impersonal remains a subject of debate.
3) Austerity --Hinduism, Buddhism and Jainism differ in the application of the yamas. Jainism says that we need to apply them to the extreme and admits of extreme austerity, but at the same time it relaxes the rules for the laity(though still strict) Buddhism takes the middle path, still demands a strong moral life, but allows for some things like eating meat. Hinduism is the most liberal in application of the yamas, admitting of kama and artha as also legitimate pursuits of life.
4) Scriptures --- This is the most biggest difference, Hindu, Buddhist and Jains do not share scriptures, they have their own 2000 year tradition of of scriptures and canons. That said, when I read them, I read very similar things.
Are Buddhism and Jainism different religions from Hinduism? Absolutely they are. The core difference is Hinduism is the Vedic religion and the Buddhists and Jains did not want be be a part of the Vedic religion, they denied the authority of the Vedas of the Brahmin caste, as well as sacraments of the Vedic tradition, therefore they definitely are different religions. However, they are so close to one another, I I have no objection practising a Buddhist meditation technique or reading Jain scriptures to enrich my own Hindu religion.
I have not looked at Sikhism here, because I consider Sikhism to be a sect of Hinduism.
