I've been thinking about this ever since I got a Legion ring (which gives the user the LOSH keyword) and it's coming up more with the new Lantern rings... except for a small handful of figures and several negative aspects, what benefits would giving a keyword to a figure (after the start of the game) do? I know there are some figures that can affect all figures with a certain keyword (some positively like Professor X with the X-Men, some negatively like a Sentinel or two against the X-Men)... but except for those rare few, the only benefit I could see would be if you could make it a theme team by doing so (which you can't).
Can anyone shed some light on what benefit(s) I'm missing?

Keywords are crucial to triggering -dependent bonuses and effects.

I'd have to dig around regarding how ATAs are treated by these characters-- since they weren't assigned them during team building-- but the overall benefits of gaining keywords post-build phase are situational, at best.

They would not get any use of the ATA.

Thanks!

I can see D10th Nightwing seeing a return to play with the GLC Ring, as he can gain the keyword pre-relic assignment (+1 to all CVs sans damage; a 20 DV up close isn't too shabby).

That's what I figured, but since this has only been an option with merge/split before now, what is the point of adding a keyword to a figure that doesn't possess it naturally? All I can think of are negatives (eg. those figures who affect all opposing figures with the same keyword) and the few positives (eg. GSX Prof X with the X-Men keyword) only apply to keywords that won't apply with these objects (LOSH ring, Yellow Lantern ring, etc).

Now it would be kind of cool if I'm wrong about the theme team aspect and they could join a theme team for the purpose of using theme team probs... but based on the new rules, I think they would need a theme team beforehand (which would negate all of those as the only other keyword that a non-LOSH would have that the LOSH have would be Future and that wouldn't give any probs). I'm still hoping there's a positive to making the point of giving a keyword to a figure after the start of the game.

The positives are that a lot of game effects care about keywords. At a brief glance through some of the Legion of Super-Heroes figures:

Cosmic Boy #001 has a power: ELECTED LEADER: Cosmic Boy can use Leadership. When he does, all characters with the Legion of Super Heroes keyword are considered a lower point value.

Gates #022 has: TELEPORT DISKS: Gates can use Phasing/Teleport. Gates can carry up to four friendly characters, regardless of their speed symbols. If all carried characters share a keyword with him, don't modify his speed value from the Carry ability.

Cosmic Boy #201 has: LONG LIVE THE LEGION!: Cosmic Boy can use Perplex and Leadership. When he uses Leadership, on a result of 6 he may also remove a token from any adjacent friendly character with the Legion of Super Heroes keyword.

Lots of other characters have powers that only work with someone who shares a keyword with them.

Those are actually really good examples (and gives better use to the Flight Ring when I build a team with it), but that just reinforces what I had said before... the only benefits to adding a keyword are figure-specific and only help in limited options with specific figures (so it can be seen as a Cosmic Boy booster, but Gates would also work with any Future figure regardless). What about the Lantern rings? I can't think of any green or yellow lantern corps figure that has any power like that, but I'm hoping they have several in the set that can optimize that feature of the rings... I hope.

Um, there are plenty of ways it matters. Any replacement would be relevant...morph, weapon swap, revert, alter ego, etc.
Solo Adventure and any effects which bring characters into the game (Madrox, Zuvembie, the Leader, etc.) also were and have been relevant.

I think you are expecting too much out of keywords, the Ring is 5pts and gives you: the Legion of Super Heroes keyword, "wing symbol", and may carry friendly characters regardless of their speed symbol. This character can use Defend and Toughness.

So you have the wing symbol, can carry regardless of their speed symbol (which is huge if you are playing a Legion team), and you have no less than 2 good defensive powers. I think you should stop worrying about the situational use of granting a character a particular keyword and think about all the other cool things the Rings give you.

I'm not thinking the keywords should have any more or less... I just wasn't sure if I missed something big that would be a big reason to give someone a keyword or if it's just a novelty thing ("Join the Legion!") that has only a few instances where it makes a difference.
I'm glad I got verification that I wasn't missing anything.