I love pso1 annd pso2 gameplay for different reasons, but the biggest thing that makes me heavily prefer pso1 is the level of engagement that you need to excel in the game. In pso2 the enemies die super fast, and they frankly have terrible passive AI. 99% of the game feels like a joke that you can button mash through like dynasty warriors. That aspect makes the smooth, fast-paced and tight combat feel unnecessary to me. Why am I so powerful when enemies die so easily and barely fight back?
Compare that to PSO1 where enemies on average took more hits, were far more aggressive (who remembers getting knocked down just to get surrounded by beasties who would just attack the air above you waiting for you to stand back up?), there was far greater emphasis on your positioning relative to the monsters which could make or break each encounter, and there was a lot more risk management when it came to ATA and choosing between accuracy, power, or risky special with the possibility of a big reward.
But honestly the main downer for me that I wish they'd improve in pso2 (fingers crossed for NG) is just how complicated things have gotten, it feels like pso2 has a bunch of knock off systems that past games did better. PSO1 and PSU had armor + unit slots, pso2 replaced single armor with a 3 piece unit set and replaced units with affixing (which needs to die imo). Affixing alone is a hassle despite being necessary for character strength progression (not to be confused with necessary to play). PSP2 has skills that you could pick, remove, and reassign at will rather than a skill tree that's exclusive to a single class that you can't just redo whenever, and skills were shared between classes instead of being exclusive to one. PSO1 had great character weapon spreadsheets; If you were a hunter you could use any melee weapon (and depending on your character, some special stuff that was fully functional like the holy ray) instead of being stuck with just 3 like pso2 where you have to level up a whole new class just because you want to use a TMG. PSP2 also had a system where you could just choose the weapons you wanted to use on ANY class, you could legit be a hunter that only used ranged and tech weapons if you wanted. But the biggest downer is the lack of trading. Double premium is needed for trading AND you can't just trade anything you want. I know some people don't particularly mind that, but for a lot of people it's a pretty big let down considering what you were able to do in the series beforehand and how it didn't carry over. Those aren't the only ones but this point is getting kinda long lol.
But there is plenty of reason to want a remaster or a remake of pso1 with some tweaks, even as a side project instead of a whole big thing. The biggest factor being that pso2 is a vastly different game from anything else in the series, and lots of vets would love to go back to the type of game that got them into the series with the same focus as before. Some people would even prefer it. But that's also the biggest downside as to why it will never happen. Aside from the cost sega would have to eat to make the project a reality, even if they just make it a side game with ONLY local co-op/split screen/peer to peer connections you still have to consider the fact that a lot of players would essentially dip out/hiatus to play that game instead, and sega isn't trying to spend money to hurt their main money maker.