What if there is no visible BTC trading pair on site? Let say one of the feature is to vote on the dapp, but in order to vote you need xx amount of utility token of the dapp, so users gonna need to deposit BTC to sort of like buying the utility token, in general does this automatically be considered a "crypto exchange" as well?
Well, while your platform might not primarily be a crypto exchange, but in your example if your platform is going to handle the exchanging of assets and not relaying it to other exchanges, then technically your platform still has an exchange feature. But then again, I think you're better off asking local authorities for a specific answer. Whatever guideline may apply in my country might not apply to yours.
In most of the countries what follows traditionally gets applied to modern things. In my country for example, gambling is not allowed and any sort of it may be, its just illegal. So even if I go and play on William Hills then its illegal in my country. In similar fashion, trading works here. Since trading is completely legal, we have seen most of the crypto exchanges arising out of no where and making this as base. They are applied with same rules as that of traditional exchanger (stock exchanges etc.) You have to report the authorities before you open up the exchanger.
It's banking has to be transparent and legally you must posses shop act license to run the same. For example, have ABC trading Co. registered in first place under which whole game will played. And of course you have to report all the earnings as well.