About the medium-big LAN idea, with nice money and sponsors, that would require quite a large prize pool and organization, both which are not going to happen. Also there are no sponsors in quake as a game anymore, and tbh right now I don't think there's more than 3 players getting flown for free.

About retired q3 legends, they stopped playing and competing for reasons that remain to this day -since they still aren't playing-, so would be quite hard to get them back playing frequently. Job and life obligations, little money here, few events, arrive home tired, got bored, the game takes too much energy and focus to be played.
They simply won't massively come back for the chance of winning $100, a quite unrealistic chance as they will be destroyed by current players. Maybe will if that amount is offered at them directly for participating.

Finally, even if you manage to convince them I think mixing retired players with active players is not that great of an idea. Someone that hasn't played in 12 years could even be defeated by cmss (1200 elo level); and even with months practice the game has evolved too much for them to catch up in order to really compete at the current level. Matches like that just aren't going to be good.

What would be spectacular is having a LAN with only classic names that haven't played in years, but is entirely impossible.



With all that said in the end your cup idea would pretty much be a QL online cup (hundreds a year nowadays) with the only difference being that it is in Q3: Same players, maybe with two old school additions that you manage to convince that will not play seriously/compete and that will lose all matches, same winner, almost same maps, in a game that's 99% the same.

I see there's a point in doing such Q3 cup, because at least it's not 100% QL, with different netcode,some gameplay changes, some different maps, internal map changes, some old names and the game looks cooler too, but not that much point since it's not really thaaaat much different than the usual QL.



The best ways to doing something similar to what you want are the following, ordered from easiest to most difficult. Personally I quite like the third option:
1. What faceit did: a QL cup inviting some old school active players that haven't retired entirely (pretty much everyone that won or got second place in something that matters in QL), mixed with others that stopped playing lets say 7 years ago (jibo) and maybe one or two that really retired 12 years ago. The results are obvious, just with some guest stars.
2. To host the said cup in the fifth paragraph. Q3, anyone can participate or can restrict it to non QL noobs, maybe some different maps, maybe get some retired players to embarrass themselves. The current plan.
3. Host that same cup in an altogether different game than Q3/QL. For example a Q4 revival cup, anyone can play (convince tox to participate as well as whoever is the current q4 player). Would be fantastic actually.
4. Get two classic retired players to do an exhibition in the game they used to play (QW, Q2, Q3, Q4, PK). While reviving 2002 is impossible at least a one 2002 match would be quite good.. Problem is the level would be mediocre and no competition. There won't be stompings and it will actually be a Legends (retired) match.
5. Doing a 4-man online Legends cup in the game they played (Q3, Q4, etc.) that only features retired players of around the same level and inactivity. It's doing faceit legends cup in a better way, as faceit accepted players that shouldn't have been there and in not the exact same game they played Problems are level of gameplay and organization, due that getting 4 really inactive really notorious players to play and of the same continent is going to be quite hard.
5. Hosting a small LAN, the same as 4 or 5 but on site.
6. A full fledged LAN of a game that isn't QL (Q3, Q4, PK) and that everyone can participate, trying to invite retired legends. This is what the QW guys do in their annual LAN.

The other alternative is convincing individual retired players to play QL actively, as this is the current active game, in any of the following fashions:
1. A not pro anymore player, rarely active, to play cups again. Tox for example.
2. Same with a really retired player that could potentially come back to challenging good players, maybe even play cups in some time. Zamuz.
3. Convincing some really old school retired player to occasionally play QL, a bit active in pub play. It's pretty much impossible to get them to actually compete if they haven't really played in more than 12 years. Lexer for example.