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I think we should come up with better mining rules that are not complicated like signet and not as vulnerable like testnet3.

True. The big question is: what should be the new consensus rules of testnet4?

Because the story is that developers already tried to make a better version of the testnet, and they came up with signet. Which means, that in 2020, nobody had any good idea, how to make a good test network, without creating a permissioned network, where only creators could mine new blocks.

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new testnet network (not Signet)

I know it is not about signet, but about testnet4, but I guess those two networks will be very similar. In general, if nobody has any good idea, then the status quo is what happens. Which means, that if nobody will invent anything better, then testnet4 will be the same as testnet3, but with a different Genesis Block, and without blockstorms. So, basically, it will be just like signet, but with signet challenge set to OP_TRUE.

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it's not easy to obtain testnet3 coins these days

This is not a bug, it is a feature. In case of the network reset, we may lose one important property: having a chain with a lot of halvings. And this is a problem, because the amount of test coins is what directly represents your ability of publicly sharing your tests. Which means, that if you have 50 tBTC, then you can push 5 GB of test cases. So, the amount of test coins is not something, that should correspond to the real values, used in mainnet. It is mainly about anti-spam features, and preventing people from flooding the network with their data.

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On top of that, recent Runes hype makes some people decide to add arbitrary data to testnet3 blockchain

This is the reason, why I agree with Garlo Nicon, that the new testnet should preserve existing halvings. So, even if all existing coins will be burned, then the block reward should not start from 50 tBTC, but rather from 0.01220703 tBTC, which we have today. Because the amount of coins you have, is what decides about your ability to push data. Which means, that if testnet4 will start from 50 tBTC, then each and every miner will get an ability to push 5 GB of data, and the new network will be flooded with data pushes from the day one, instead of being used to test payments.