Perhaps they could have an equipment grid.

Why do you want better locomotives? Do you like the challenge of researching more to unlock them or do you just want faster trains? If you want faster trains, you can simply enable cheatsy locomotives and set a higher top speed and/or acceleration in the mod settings.

I’m frankly not even sure what the tech tree should look like for better locomotives. What would be sensible tech requirements for that? Simply adding more electric motors would be boring.

Electric locomotives can already have equipment grids, but that requires either Bob's Vehicle Equipment or Krastorio 2. Unfortunately, neither mod is available for Factorio 2.0 yet. I hope that Bob's Vehicle Equipment will be released for 2.0 in the near future.

I like to research them, but mostly I like the mk1 and mk2 locomotives to be not faster, but heavier and more powerful, and tougher. They are meant to penetrate where bugs might be on the tracks. I do not see any point in just setting a higher top speed, too cheesy. I guess we'll have to leave it to Bob then to get equipment grids on them.

I guess what I'm really looking for is variants, that I can further mod and chose between.

What kind of variants are you imagining?

Well, like locomotives that are heavier, with more acceleration but not more top speed, so they pull larger trains better and push through the bugs. More health, to survive a few hits. And of course, the equipment grid, I'd envision 6x6 for a mark1, 6x8 for a mark2. That way one could mount a couple shields, and a couple lasers to fight back. That's how I'd plan to mod the variants. I can poke and hack a bit on some mods, but I admit I cannot do the code wholesale.
Having said that, I really enjoy modding Factorio and might like to learn to make mods myself.

I really like the way you've done electric trains overall, really like the zero UPS consumption. Charging stations is a logistical thing players like to do.