It's kind of similar to staking your BTC but using your holding wallet. Such features would be sick as it would make it easy for a lot of users to swap their coins back and forth without the need of a centralized exchange.

Though the reason why I still won't prefer this is that, if we are Bitcoin holders and wish to make more profit on the long-term base, we might miss out on the market. You know how volatile the market is.

Let's take for instance, when BTC broke the 90-100k mark, it was so surprising as it happened overnight. Imagine using such features and end up not gaining in the bull? Let's say you had BTC when the price was $48k and you use such features to hold your coin in USDT. Then the price moved from $48k to $100k, you would have missed out on the bull and if you finally unlock your coins back to BTC at $100k, you would have less BTC because it's going to convert back to you at the current market price.

Then the reverse would be the case if you use the same feature and the price breaks down on a bear, from $48k to $30k, when you finally convert it back, you would have more BTC worth than you have held. But what's the probability of BTC going down to such a price than moving forward? It might be a nice feature, but I don't think it would be profitable the way you think of it.

No transfer fees. Hold the wallet on your desktop. Fast moves. Complete control.

Bitcoin only goes up or down. This only moves Bitcoin in or out as you desire.

I still don’t get your point OP, if you don’t want to see the balance of your wallet go down maybe when there’s a dip in the Bitcoin market,then you can just turn off the fiat equivalent display for the UI, almost every wallet should have the feature and if it’s back to a satisfactory level then you can turn it back on..
I think almost every wallet that supports multiple input address can freeze UTXOs, I’m sure of Electrum and Blue-wallet as well..

It would be nice to have this, but I don't know if this is possible. In the first place, will there ever be a wallet which supports fiat that's completely under your control? I doubt it. At most, you'll have to use a stablecoin.

Secondly, will there ever be a crypto-to-fiat conversion and vice versa without any fees to pay or spread to cover? In certain centralized exchanges, there are P2P crypto-to-fiat and vice versa exchange features that have zero fees, except perhaps those that your banks charge you. But you can't have complete control in this setup. In decentralized setups like Bisq, you will pay fees.

I guess the function you have in mind isn't existent, at least not yet. But it seems you oversimplify Bitcoin's movements. Even if certain wallets or exchanges support this, it doesn't mean you'll make profit out of Bitcoin's ups and downs and your ability to instantly get in or out of Bitcoin.

Changelly serve more like a bridge between the wallet and the exchange.
It finds the best rate and after converting send to your account.

Its possible but not worth the risk because it's the same as having a third party access to your coins.
Complexities most times bring about vulnerabilities.

You should avoid Changelly, it's KYC exchanger, there have been a lot of scam accusations against them, I wouldn't consider them trustworthy for anything.

However, for trading, I would prefer to choose exchange platforms, and wallets are for holding coins. As ABCbits said, you can't avoid the TX fee. Use an application or widget that will send you notifications when Bitcoin reaches a certain value.

This sounds like very useful and money saving feature. Basically you are talking about system inside wallet that lets you temporarily connect value of your Bitcoin to regular currency like USD without actually changing it to stablecoin. With this we protect ourself from big price changes and avoid paying fees to move money around. With clicking button you can easily switch between seeing value of your Bitcoin in Bitcoin or in USD based on current exchange rate. This feature would be especially helpful for people who want to keep track of their Bitcoin in relation to regular currencies without wasting money on unnecessary transfer fees.

So you are talking about immediate swap between BTC and stablecoins? This seems like a cool idea, but it doesn't work in reality.
Swapping BTC to stablecoins is just selling one crypto for another. Most multi-coin crypto wallets offer such feature, but it is provided by third parties. Exodus wallet has a swapping feature, but AFAIK it's provided by Changelly and some Exodus users aren't happy with this feature.
If you are talking about converting BTC to stablecoins WITHOUT an actual transfer, this doesn't make sense to me. You are just measuring the current BTC price in stablecoins. This doesn't change anything.

What is UTXO?

All I want is a wallet with Bitcoin and USDC... and the ability inside the wallet to switch between the two at will.

I'm sure that there are a lot of others who would like this feature, as well.

Since Changelly has it one way or another, it shows that it can be done. Maybe it isn't worth the doing. And I am not tech savvy enough to do it. But it is there in Changelly, and probably other swap wallets.