This has the same problem that a lot of fan-made planeswalkers have, which is that the + ability can easily end up impossible to activate (say after a Terminus), or in a state where the only creatures to target are the opponents'. Wizards have never printed a planeswalker who needs a creature on your side of the table in order to gain loyalty, for precisely this reason.

I like making a Lord of Extinction as an ultimate though. It's using a lot of words on its colours - perhaps all those colours aren't necessary - but details.

This one is easy to solve, though. If you change it to "Put a +1/+1 counter on a creature you control", you're fine. You lose some fun interactions... but those interactions didn't appear on this card. You can also do a few cheezy things, like put a +1/+1 counter on a White Knight, which this card won't do currently.

You also lose the opportunity for opponents to respond with things like Lightning Bolt or Last Gasp. I don't think removing targetedness from an ability is a very good workaround here. You don't see any planeswalkers like that either, where you think "Why isn't this a targeted ability? Ohhh."

I think it'd be better to say "up to one target creature", but even better would be to find a + ability that doesn't raise that question.

You don't, but I highly expect that to begin happening in the future. There are only so many minor non-targeted abilities.